Layout containers with fullscreen child correctly

When a container that includes a child that's fullscreen gets
re-arranged also adjust the saved width/height of the fullscreen window
so that on fullscreen exit the layout is correct.

Fixes #3547

include/sway/tree/container.h

@@ -87,6 +87,7 @@ struct sway_container {
 	double width, height;
 	double saved_x, saved_y;
 	double saved_width, saved_height;
+	bool saved_dims;
 
 	// These are in layout coordinates.
 	double content_x, content_y;