[Fix] (MG_State, MG_Impl): a transform feedback name is only an object once it is bound

glIsTransformFeedback answered GL_TRUE for any name glGenTransformFeedbacks had handed
out. A generated name is reserved but does not denote an object until the first
glBindTransformFeedback (GL 4.6 core 13.2.1) - the same rule the other object types
follow - and KHR-GL40.api.coverage checks exactly the window in between.

The two questions are now asked separately: whether a name may be bound or deleted
(reserved, which is what the delete and bind paths need) and whether it is an object
(reserved and bound at least once).

MobileGL/MG_State/GLState/Core.cpp

@@ -798,9 +798,16 @@ namespace MobileGL::MG_State {
             if (index == m_boundTransformFeedback) return;
             SaveBoundTransformFeedbackState();
             m_boundTransformFeedback = index;
+            m_transformFeedbackObjects[index].everBound = true;
             RestoreBoundTransformFeedbackState();
         }
 
+        Bool GLContext::IsTransformFeedbackObject(Uint index) const {
+            if (index == 0 || !m_transformFeedbackNames.IsValid(index)) return false;
+            const auto it = m_transformFeedbackObjects.find(index);
+            return it != m_transformFeedbackObjects.end() && it->second.everBound;
+        }
+
         void GLContext::MarkTransformFeedbackObjectForDeletion(Uint index) {
             if (index == 0 || !m_transformFeedbackNames.IsValid(index)) return;
             // Deleting the bound object reverts to the default one (GL 4.6 core 13.2.1);