_cairo_xcb_surface_ensure_picture: Check fallback

When an XCB surface's fallback member is not NULL, we did some rendering that
the X server couldn't do for us and thus did that internally in cairo instead.
This means the X drawable's content is out of date and should not be used for
drawing anything.

This adds an assert which checks for that situation.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-xcb-surface-render.c

@@ -301,6 +301,7 @@ static void
 _cairo_xcb_surface_ensure_picture (cairo_xcb_surface_t *surface)
 {
     if (surface->picture == XCB_NONE) {
+	assert (surface->fallback == NULL);
 	surface->picture = _cairo_xcb_connection_get_xid (surface->connection);
 	_cairo_xcb_connection_render_create_picture (surface->connection,
 						     surface->picture,