Shaders: properly check for EXT_gpu_shader4 if ObjectId is enabled.

I was checking many things but not this one.

src/Magnum/Shaders/FlatGL.cpp

@@ -111,6 +111,8 @@ template<UnsignedInt dimensions> typename FlatGL<dimensions>::CompileState FlatG
     #endif
 
     #ifndef MAGNUM_TARGET_GLES
+    if(configuration.flags() >= Flag::ObjectId)
+        MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::EXT::gpu_shader4);
     if(configuration.flags() >= Flag::UniformBuffers)
         MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::ARB::uniform_buffer_object);
     #endif

src/Magnum/Shaders/MeshVisualizerGL.cpp

@@ -103,6 +103,8 @@ void MeshVisualizerGLBase::assertExtensions(const FlagsBase flags) {
     #endif
 
     #ifndef MAGNUM_TARGET_GLES
+    if(flags >= FlagBase::ObjectId)
+        MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::EXT::gpu_shader4);
     if(flags >= FlagBase::UniformBuffers)
         MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::ARB::uniform_buffer_object);
     #endif

src/Magnum/Shaders/PhongGL.cpp

@@ -126,6 +126,8 @@ PhongGL::CompileState PhongGL::compile(const Configuration& configuration) {
     #endif
 
     #ifndef MAGNUM_TARGET_GLES
+    if(configuration.flags() >= Flag::ObjectId)
+        MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::EXT::gpu_shader4);
     if(configuration.flags() >= Flag::UniformBuffers)
         MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::ARB::uniform_buffer_object);
     #endif