glthread: track all matrix stack depths

This just tracks matrix stack depths in MatrixStackDepth and everything
else here is needed to make it correct.

Matrix stack depths will be returned by glGetIntegerv without synchronizing.

Display lists will be handled by a separate commit.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8297>