Shaders: WebGL doesn't allow uploading an UBO shorter than hardcoded.

Also fixes a WebGL error in the tests. I suspect it might fail elsewhere
as well. Nevertheless, this still doesn't mean that it would be
impossible to use dynamic joint count with UBOs -- simply pad the UBO in
that case.

src/Magnum/Shaders/Test/FlatGLTest.cpp

@@ -3191,6 +3191,11 @@ template<FlatGL2D::Flag flag> void FlatGLTest::renderSkinning2D() {
         Matrix3{Math::IdentityInit},
     };
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    if(flag == FlatGL2D::Flag::UniformBuffers && data.jointCount > Containers::arraySize(jointMatrices))
+        CORRADE_SKIP("Uploading an uniform buffer smaller than the size hardcoded in the shader is an error in WebGL.");
+    #endif
+
     GL::Buffer buffer{vertices};
 
     GL::Mesh mesh{MeshPrimitive::TriangleStrip};
@@ -3317,6 +3322,11 @@ template<FlatGL2D::Flag flag> void FlatGLTest::renderSkinning3D() {
         Matrix4{Math::IdentityInit},
     };
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    if(flag == FlatGL3D::Flag::UniformBuffers && data.jointCount > Containers::arraySize(jointMatrices))
+        CORRADE_SKIP("Uploading an uniform buffer smaller than the size hardcoded in the shader is an error in WebGL.");
+    #endif
+
     GL::Buffer buffer{vertices};
 
     GL::Mesh mesh{MeshPrimitive::TriangleStrip};

src/Magnum/Shaders/Test/MeshVisualizerGLTest.cpp

@@ -4804,6 +4804,11 @@ template<MeshVisualizerGL2D::Flag flag> void MeshVisualizerGLTest::renderSkinnin
         Matrix3{Math::IdentityInit},
     };
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    if(flag == MeshVisualizerGL2D::Flag::UniformBuffers && data.jointCount > Containers::arraySize(jointMatrices))
+        CORRADE_SKIP("Uploading an uniform buffer smaller than the size hardcoded in the shader is an error in WebGL.");
+    #endif
+
     GL::Buffer buffer{vertices};
 
     GL::Mesh mesh{MeshPrimitive::Triangles};
@@ -4937,6 +4942,11 @@ template<MeshVisualizerGL3D::Flag flag> void MeshVisualizerGLTest::renderSkinnin
         Matrix4{Math::IdentityInit},
     };
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    if(flag == MeshVisualizerGL3D::Flag::UniformBuffers && data.jointCount > Containers::arraySize(jointMatrices))
+        CORRADE_SKIP("Uploading an uniform buffer smaller than the size hardcoded in the shader is an error in WebGL.");
+    #endif
+
     GL::Buffer buffer{vertices};
 
     GL::Mesh mesh{MeshPrimitive::Triangles};

src/Magnum/Shaders/Test/PhongGLTest.cpp

@@ -4033,6 +4033,11 @@ template<PhongGL::Flag flag> void PhongGLTest::renderSkinning() {
         Matrix4{Math::IdentityInit},
     };
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    if(flag == PhongGL::Flag::UniformBuffers && data.jointCount > Containers::arraySize(jointMatrices))
+        CORRADE_SKIP("Uploading an uniform buffer smaller than the size hardcoded in the shader is an error in WebGL.");
+    #endif
+
     GL::Buffer buffer{vertices};
 
     GL::Mesh mesh{MeshPrimitive::TriangleStrip};