[broadway] Send diffs as bilevel rgba instead of true diff

The true diff only works if the destination keeps perfect 32bit
canvas data, which is not always true. So, instead we send only
changed pixels, masking the others to 0 via alpha 0.

gdk/broadway/broadway.h

@@ -39,14 +39,6 @@ void            broadway_client_put_rgba        (BroadwayClient *client,
 						 int             h,
 						 int             byte_stride,
 						 void           *data);
-void            broadway_client_put_delta_rgb   (BroadwayClient *client,
-						 int             id,
-						 int             dest_x,
-						 int             dest_y,
-						 int             w,
-						 int             h,
-						 int             byte_stride,
-						 void           *data);
 void            broadway_client_copy_rectangles (BroadwayClient *client,
 						 int             id,
 						 BroadwayRect   *rects,