mesa: Make more consistent use of _mesa_is_{user,winsys}_fbo()

mesa: Make more consistent use of _mesa_is_{user,winsys}_fbo()

A lot of code was still differentiating between between winsys and
user fbos by testing the fbo's name against zero.  This converts
everything in core mesa, the state tracker, and src/mesa/program over
to use _mesa_is_user_fbo() and _mesa_is_winsys_fbo().

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>