Make the OpenGL context associated with the given display current for
the calling thread.
If there is a current target bitmap which belongs to a different OpenGL
context, the target bitmap will be changed to NULL.
Normally you do not need to use this function, as the context will be
made current when you call al_set_target_bitmap(3) or
You might need if it you created an OpenGL "forward compatible" context.
Then al_get_backbuffer(3) only returns NULL, so it would not work to
pass that to al_set_target_bitmap(3).