glthread: add marshal_call_after and remove custom glFlush and glEnable code

Instead of implementing marshalling manually, this XML property allows us
to insert additional code into code-generated functions.

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