glamor: handle NULL source picture

COMPOSITE_REGION() can pass NULL as a source picture, make sure we
handle that nicely in both glamor_composite_clipped_region() and
glamor_composite_choose_shader().

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=101894


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit bd353e9b)