meta: Save and restore the framebuffer using gl_framebuffer instead of GL API object...

meta: Save and restore the framebuffer using gl_framebuffer instead of GL API object handle

Some meta operations can be called recursively.  Future changes (the
"Don't pollute the ... namespace" changes) will cause objects with
invalid names to be used.  If a nested meta operation tries to restore
an object named 0xDEADBEEF, it will fail.

This also fixes another latent bug in meta.  In a multithreaded,
multicontext application, one thread can delete an object that is bound
in another thread.  That object continues to exist until it is unbound
(i.e., its refcount drops to zero).  Meta unbinds objects all over the
place.  As a result, the rebind in _mesa_meta_end could fail because the
object vanished!

See https://bugs.freedesktop.org/show_bug.cgi?id=92363#c8.

Using _mesa_reference_<object type> to save and restore the objects
prevents the refcount from going to zero.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92363
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>