mesa: Implement mutex/locking around texture object reference counting.

mesa: Implement mutex/locking around texture object reference counting.

Use new _mesa_reference_texobj() function for referencing/unreferencing
textures.  Add new assertions/tests to try to detect invalid usage of
deleted textures.

cherry-picked from master (9e01b915f1243a3f551cb795b7124bd1e52ca15f)