glthread: track for each VAO whether the user has set a user pointer

glthread: track for each VAO whether the user has set a user pointer

This commit mainly adds basic infrastructure for tracking vertex array
state.

If glthread gets a non-VBO pointer, this commit delays disabling
glthread until glDraw is called. The next will change that to "sync"
instead of "disable".

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3948>