Weston releases buffers almost immediately after they're done, which
means that GTK+ doesn't use a temporary surface and instead paints
directly onto the SHM backing store that Weston will use.

Normally, after painting to the temporary surface, GTK+ *replaces*
the existing backing surface with CAIRO_OPERATOR_SOURCE. However,
if we immediately paint to the backing surface, it might have junk
from the last paint in it. So clear out the backing surface whenever
somebody calls begin_paint_region().

Maybe we should just always use the temporary surface like the X11
codepath, since that prevents us from having to do weird things like
this, but oh well.