Mesh: set VAO id to 0 when the extension is not available.

Also expect this case in test.

src/Mesh.cpp

@@ -270,7 +270,7 @@ void Mesh::initializeContextBasedFunctionality(Context& context) {
     #endif
 }
 
-void Mesh::createImplementationDefault() {}
+void Mesh::createImplementationDefault() { _id = 0; }
 
 void Mesh::createImplementationVAO() {
     /** @todo Get some extension wrangler instead to avoid linker errors to glGenVertexArrays() on ES2 */

src/Mesh.h

@@ -411,7 +411,7 @@ class MAGNUM_EXPORT Mesh: public AbstractObject {
          * @brief OpenGL mesh ID
          *
          * If @extension{APPLE,vertex_array_object} is not available, returns
-         * 0.
+         * `0`.
          */
         GLuint id() const { return _id; }
 

src/Test/MeshGLTest.cpp

@@ -53,7 +53,15 @@ void MeshGLTest::construct() {
         const Mesh mesh;
 
         MAGNUM_VERIFY_NO_ERROR();
-        CORRADE_VERIFY(mesh.id() > 0);
+
+        #ifndef MAGNUM_TARGET_GLES
+        if(Context::current()->isExtensionSupported<Extensions::GL::APPLE::vertex_array_object>())
+        #elif defined(MAGNUM_TARGET_GLES2)
+        if(Context::current()->isExtensionSupported<Extensions::GL::OES::vertex_array_object>())
+        #endif
+        {
+            CORRADE_VERIFY(mesh.id() > 0);
+        }
     }
 
     MAGNUM_VERIFY_NO_ERROR();
@@ -72,7 +80,15 @@ void MeshGLTest::constructMove() {
     const Int id = a.id();
 
     MAGNUM_VERIFY_NO_ERROR();
-    CORRADE_VERIFY(id > 0);
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::APPLE::vertex_array_object>())
+    #elif defined(MAGNUM_TARGET_GLES2)
+    if(Context::current()->isExtensionSupported<Extensions::GL::OES::vertex_array_object>())
+    #endif
+    {
+        CORRADE_VERIFY(id > 0);
+    }
 
     Mesh b(std::move(a));
 
@@ -84,7 +100,16 @@ void MeshGLTest::constructMove() {
     c = std::move(b);
 
     MAGNUM_VERIFY_NO_ERROR();
-    CORRADE_VERIFY(cId > 0);
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current()->isExtensionSupported<Extensions::GL::APPLE::vertex_array_object>())
+    #elif defined(MAGNUM_TARGET_GLES2)
+    if(Context::current()->isExtensionSupported<Extensions::GL::OES::vertex_array_object>())
+    #endif
+    {
+        CORRADE_VERIFY(cId > 0);
+    }
+
     CORRADE_COMPARE(b.id(), cId);
     CORRADE_COMPARE(c.id(), id);
 }