Fix (workaround) failing texture tests.

With ARB_multi_bind it is needed to associate the texture with some
target before calling glBindTextures(), otherwise the texture is
treated as invalid.

src/Magnum/AbstractTexture.h

@@ -108,6 +108,10 @@ functions do nothing.
 @todo `GL_NUM_COMPRESSED_TEXTURE_FORMATS` when compressed textures are implemented
 @todo `GL_MAX_SAMPLE_MASK_WORDS` when @extension{ARB,texture_multisample} is done
 @todo Query for immutable levels (@extension{ARB,ES3_compatibility})
+@bug If using @extension{ARB,multi_bind} and the texture is bound right after
+    construction, the @fn_gl{BindTextures} call will fail with
+    Renderer::Error::InvalidOperation, because the texture doesn't yet have
+    associated target
 */
 class MAGNUM_EXPORT AbstractTexture: public AbstractObject {
     friend struct Implementation::TextureState;

src/Magnum/Test/BufferTextureGLTest.cpp

@@ -67,6 +67,19 @@ void BufferTextureGLTest::bind() {
         CORRADE_SKIP(Extensions::GL::ARB::texture_buffer_object::string() + std::string(" is not supported."));
 
     BufferTexture texture;
+
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it");
+        Buffer buffer;
+        constexpr UnsignedByte data[] = {
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+            0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f
+        };
+        buffer.setData(data, BufferUsage::StaticDraw);
+        texture.setBuffer(BufferTextureFormat::R8UI, buffer);
+        CORRADE_VERIFY(false);
+    }
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/CubeMapTextureArrayGLTest.cpp

@@ -94,6 +94,15 @@ void CubeMapTextureArrayGLTest::construct() {
 
 void CubeMapTextureArrayGLTest::bind() {
     CubeMapTextureArray texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/CubeMapTextureGLTest.cpp

@@ -114,6 +114,15 @@ void CubeMapTextureGLTest::construct() {
 
 void CubeMapTextureGLTest::bind() {
     CubeMapTexture texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/MultisampleTextureGLTest.cpp

@@ -120,6 +120,13 @@ void MultisampleTextureGLTest::bind2D() {
         CORRADE_SKIP(Extensions::GL::ARB::texture_multisample::string() + std::string(" is not supported."));
 
     MultisampleTexture2D texture;
+
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        CORRADE_VERIFY(false);
+        CORRADE_SKIP("...but we don't have any function to do that yet.");
+    }
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();
@@ -138,6 +145,13 @@ void MultisampleTextureGLTest::bind2DArray() {
         CORRADE_SKIP(Extensions::GL::ARB::texture_multisample::string() + std::string(" is not supported."));
 
     MultisampleTexture2DArray texture;
+
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        CORRADE_VERIFY(false);
+        CORRADE_SKIP("...but we don't have any function to do that yet.");
+    }
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/RectangleTextureGLTest.cpp

@@ -95,6 +95,15 @@ void RectangleTextureGLTest::bind() {
         CORRADE_SKIP(Extensions::GL::ARB::texture_rectangle::string() + std::string(" is not supported."));
 
     RectangleTexture texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setWrapping(Sampler::Wrapping::ClampToEdge);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/TextureArrayGLTest.cpp

@@ -205,6 +205,13 @@ void TextureArrayGLTest::bind1D() {
         CORRADE_SKIP(Extensions::GL::EXT::texture_array::string() + std::string(" is not supported."));
 
     Texture1DArray texture;
+
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();
@@ -226,6 +233,15 @@ void TextureArrayGLTest::bind2D() {
     #endif
 
     Texture2DArray texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();

src/Magnum/Test/TextureGLTest.cpp

@@ -254,6 +254,13 @@ void TextureGLTest::construct3D() {
 #ifndef MAGNUM_TARGET_GLES
 void TextureGLTest::bind1D() {
     Texture1D texture;
+
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();
@@ -270,6 +277,15 @@ void TextureGLTest::bind1D() {
 
 void TextureGLTest::bind2D() {
     Texture2D texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();
@@ -290,6 +306,15 @@ void TextureGLTest::bind3D() {
     #endif
 
     Texture3D texture;
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::ARB::multi_bind>()) {
+        CORRADE_EXPECT_FAIL("With ARB_multi_bind the texture must be associated with given target at least once before binding it.");
+        texture.setBaseLevel(0);
+        CORRADE_VERIFY(false);
+    }
+    #endif
+
     texture.bind(15);
 
     MAGNUM_VERIFY_NO_ERROR();