xdg-shell: Add set_margin request

This is used to figure out the size of "invisible" decorations, which we'll
use to better know the visible extents of the surface, which we can use for
constraining, titlebars, and more.

clients/window.c

@@ -3937,6 +3937,27 @@ window_sync_transient_for(struct window *window)
 	xdg_surface_set_transient_for(window->xdg_surface, parent_surface);
 }
 
+static void
+window_sync_margin(struct window *window)
+{
+	int margin;
+
+	if (!window->xdg_surface)
+		return;
+
+	if (!window->frame)
+		return;
+
+	margin = frame_get_shadow_margin(window->frame->frame);
+
+	/* Shadow size is the same on every side. */
+	xdg_surface_set_margin(window->xdg_surface,
+				     margin,
+				     margin,
+				     margin,
+				     margin);
+}
+
 static void
 window_flush(struct window *window)
 {
@@ -3953,6 +3974,7 @@ window_flush(struct window *window)
 						 &xdg_surface_listener, window);
 
 			window_sync_transient_for(window);
+			window_sync_margin(window);
 		}
 	}