Returns the OpenGL FBO id internally used by the given bitmap if it uses
one, otherwise returns zero.
No attempt will be made to create an FBO if the bitmap is not owned by
the current display.
The FBO returned by this function will only be freed when the bitmap is
destroyed, or if you call al_remove_opengl_fbo(3) on the bitmap.
I]Note:] In Allegro 5.0.0 this function only returned an FBO which
had previously been created by calling al_set_target_bitmap(3).
It would not attempt to create an FBO itself.
This has since been changed.