GL: make CompressedBufferImage and BufferImage::setData() work the same.

The uncompressed variant preserves the buffer contents if passed an empty
view, which was done in 5ee461bdbe (2015!)
but no corresponding change was ever done for the compressed variant. So
let's make both do the same, and actually test the code path, which
wasn't done until now either.

src/Magnum/GL/BufferImage.cpp

@@ -134,8 +134,12 @@ template<UnsignedInt dimensions> void CompressedBufferImage<dimensions>::setData
     _storage = storage;
     _format = format;
     _size = size;
-    _buffer.setData(data, usage);
-    _dataSize = data.size();
+
+    /* Keep the old storage if zero-sized nullptr buffer was passed */
+    if(!(data.data() == nullptr && data.size() == 0)) {
+        _buffer.setData(data, usage);
+        _dataSize = data.size();
+    }
 }
 
 template<UnsignedInt dimensions> void CompressedBufferImage<dimensions>::setData(const CompressedPixelStorage storage, const Magnum::CompressedPixelFormat format, const VectorTypeFor<dimensions, Int>& size, const Containers::ArrayView<const void> data, const BufferUsage usage) {

src/Magnum/GL/BufferImage.h

@@ -640,7 +640,9 @@ template<UnsignedInt dimensions> class CompressedBufferImage {
          * @param data              Image data
          * @param usage             Image buffer usage
          *
-         * Updates the image buffer with given data.
+         * Updates the image buffer with given data. Passing @cpp nullptr @ce
+         * zero-sized @p data will not reallocate current storage, but expects
+         * that current data size is large enough for the new parameters.
          * @see @ref Buffer::setData()
          * @requires_gl42 Extension @gl_extension{ARB,compressed_texture_pixel_storage}
          * @requires_gl Compressed pixel storage is hardcoded in OpenGL ES and

src/Magnum/GL/Test/BufferImageGLTest.cpp

@@ -62,8 +62,10 @@ struct BufferImageGLTest: OpenGLTester {
 
     void setData();
     void setDataGeneric();
+    void setDataKeepStorage();
     void setDataCompressed();
     void setDataCompressedGeneric();
+    void setDataCompressedKeepStorage();
     void setDataInvalidSize();
     void setDataCompressedInvalidSize();
 
@@ -94,8 +96,10 @@ BufferImageGLTest::BufferImageGLTest() {
 
               &BufferImageGLTest::setData,
               &BufferImageGLTest::setDataGeneric,
+              &BufferImageGLTest::setDataKeepStorage,
               &BufferImageGLTest::setDataCompressed,
               &BufferImageGLTest::setDataCompressedGeneric,
+              &BufferImageGLTest::setDataCompressedKeepStorage,
               &BufferImageGLTest::setDataInvalidSize,
               &BufferImageGLTest::setDataCompressedInvalidSize,
 
@@ -616,6 +620,39 @@ void BufferImageGLTest::setDataGeneric() {
     #endif
 }
 
+void BufferImageGLTest::setDataKeepStorage() {
+    const char data[]{
+        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l'
+    };
+    BufferImage2D a{
+        PixelFormat::Red, PixelType::UnsignedByte, {4, 1},
+        data, BufferUsage::StaticDraw};
+
+    a.setData(PixelStorage{}.setAlignment(1),
+        PixelFormat::RGB, PixelType::UnsignedShort, {2, 1},
+        nullptr, BufferUsage::StaticDraw);
+
+    #ifndef MAGNUM_TARGET_GLES
+    const auto imageData = a.buffer().data();
+    #endif
+
+    MAGNUM_VERIFY_NO_GL_ERROR();
+
+    CORRADE_COMPARE(a.storage().alignment(), 1);
+    CORRADE_COMPARE(a.format(), PixelFormat::RGB);
+    CORRADE_COMPARE(a.type(), PixelType::UnsignedShort);
+    CORRADE_COMPARE(a.size(), (Vector2i{2, 1}));
+    CORRADE_COMPARE(a.pixelSize(), 6);
+    CORRADE_COMPARE(a.dataSize(), 12);
+
+    /** @todo How to verify the contents in ES? */
+    #ifndef MAGNUM_TARGET_GLES
+    CORRADE_COMPARE_AS(imageData,
+        Containers::arrayView(data),
+        TestSuite::Compare::Container);
+    #endif
+}
+
 void BufferImageGLTest::setDataCompressed() {
     const char data[] = { 'a', 0, 0, 0, 'b', 0, 0, 0 };
     CompressedBufferImage2D a{CompressedPixelFormat::RGBAS3tcDxt1, {4, 4}, data, BufferUsage::StaticDraw};
@@ -678,6 +715,38 @@ void BufferImageGLTest::setDataCompressedGeneric() {
     #endif
 }
 
+void BufferImageGLTest::setDataCompressedKeepStorage() {
+    const char data[]{
+        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h',
+        'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p'
+    };
+    CompressedBufferImage2D a{
+        CompressedPixelFormat::RGBAS3tcDxt1, {8, 3},
+        data, BufferUsage::StaticDraw};
+
+    a.setData(CompressedPixelStorage{}.setRowLength(3),
+        CompressedPixelFormat::SRGB8Alpha8Astc4x4, {2, 4},
+        nullptr, BufferUsage::StaticDraw);
+
+    #ifndef MAGNUM_TARGET_GLES
+    const auto imageData = a.buffer().data();
+    #endif
+
+    MAGNUM_VERIFY_NO_GL_ERROR();
+
+    CORRADE_COMPARE(a.storage().rowLength(), 3);
+    CORRADE_COMPARE(a.format(), CompressedPixelFormat::SRGB8Alpha8Astc4x4);
+    CORRADE_COMPARE(a.size(), (Vector2i{2, 4}));
+    CORRADE_COMPARE(a.dataSize(), 16);
+
+    /** @todo How to verify the contents in ES? */
+    #ifndef MAGNUM_TARGET_GLES
+    CORRADE_COMPARE_AS(imageData,
+        Containers::arrayView(data),
+        TestSuite::Compare::Container);
+    #endif
+}
+
 void BufferImageGLTest::setDataInvalidSize() {
     CORRADE_SKIP_IF_NO_ASSERT();