Fix memleak in container_get_box

Rather than allocate a structure and expect callers to free it, take a
pointer to an existing struct as an argument.

This function is no longer called anywhere though.

include/sway/tree/container.h

@@ -295,6 +295,6 @@ bool container_is_floating(struct sway_container *container);
 /**
  * Get a container's box in layout coordinates.
  */
-struct wlr_box *container_get_box(struct sway_container *container);
+void container_get_box(struct sway_container *container, struct wlr_box *box);
 
 #endif

sway/tree/container.c

@@ -988,11 +988,9 @@ bool container_is_floating(struct sway_container *container) {
 	return container->parent == workspace->sway_workspace->floating;
 }
 
-struct wlr_box *container_get_box(struct sway_container *container) {
-	struct wlr_box *box = calloc(1, sizeof(struct wlr_box));
+void container_get_box(struct sway_container *container, struct wlr_box *box) {
 	box->x = container->x;
 	box->y = container->y;
 	box->width = container->width;
 	box->height = container->height;
-	return box;
 }