Crowd: Unbind warns if missing bge_Model uniform
internal/Graphics: Remove GL implementation data getters
internal/Graphics: Shadow states stack-allocated

If heap-allocated, we might run into issues when threads terminate.
There could be memory leaks, since they don't free the thread-local
_GL pointer. So we'll just have the states themselves be allocated
on the stack and make all the internal thread-entry functions
zero-mem their shadow state.
internal/Graphics: _ResetShadowState method added

Called by internal thread entry functions, as well as used when
initializing the internal graphics module. Removes the need for the
_InitShadowState method, so that has been removed.