[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_Impl/GLImpl/Drawing/GL_Drawing.cpp

@@ -909,8 +909,9 @@ namespace MobileGL::MG_Impl::GLImpl {
     }
 
     GLboolean IsTransformFeedback(GLuint id) {
-        // Name 0 is the default object, which glIsTransformFeedback reports as not an object.
-        return (id != 0 && MG_State::pGLContext->ValidateTransformFeedbackName(id)) ? GL_TRUE : GL_FALSE;
+        // Name 0 is the default object, and a name glGenTransformFeedbacks handed out only
+        // becomes the name of an object once it has been bound.
+        return MG_State::pGLContext->IsTransformFeedbackObject(id) ? GL_TRUE : GL_FALSE;
     }
 
     // glDrawTransformFeedback[Stream][Instanced]: replays the vertices the named object

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);

MobileGL/MG_State/GLState/Core.h

@@ -274,6 +274,9 @@ namespace MobileGL {
                 // A name glGenTransformFeedbacks handed out and glDeleteTransformFeedbacks
                 // has not taken back. Name 0 is always valid.
                 Bool ValidateTransformFeedbackName(Uint index) const;
+                // What glIsTransformFeedback reports: a generated name only becomes the name
+                // of an object once it has been bound at least once (GL 4.6 core 13.2.1).
+                Bool IsTransformFeedbackObject(Uint index) const;
                 void BindTransformFeedbackObject(Uint index);
                 void MarkTransformFeedbackObjectForDeletion(Uint index);
                 Uint GetBoundTransformFeedbackName() const { return m_boundTransformFeedback; }
@@ -344,6 +347,7 @@ namespace MobileGL {
                     Uint64 inputPrimitives = 0;
                     Uint64 recordedVertices = 0;
                     Bool hasCompletedSpan = false;
+                    Bool everBound = false;
                 };
                 void SaveBoundTransformFeedbackState();
                 void RestoreBoundTransformFeedbackState();