compositor: damage the whole buffer on viewport src change

wp_viewporter protocol doesn't seem to say anything about damage, but
Firefox assumes that wp_viewport::set_source alone is enough to damage
the whole surface, and that assumption kinda makes sense, so let's do
that.

types/wlr_compositor.c

@@ -208,8 +208,12 @@ static void surface_update_damage(pixman_region32_t *buffer_damage,
 	pixman_region32_clear(buffer_damage);
 
 	if (pending->width != current->width ||
-			pending->height != current->height) {
-		// Damage the whole buffer on resize
+			pending->height != current->height ||
+			pending->viewport.src.x != current->viewport.src.x ||
+			pending->viewport.src.y != current->viewport.src.y ||
+			pending->viewport.src.width != current->viewport.src.width ||
+			pending->viewport.src.height != current->viewport.src.height) {
+		// Damage the whole buffer on resize or viewport source box change
 		pixman_region32_union_rect(buffer_damage, buffer_damage, 0, 0,
 			pending->buffer_width, pending->buffer_height);
 	} else {