util/box: Add wlr_box_intersects

wlr_box_intersection generates a new box based on the intersection of
two boxes. Often we simply want to know *if* two boxes intersected,
which we can answer much cheaper.

Add wlr_box_intersects, in similar vein as wlr_box_contains_box but
returning true for any overlap.

include/wlr/util/box.h

@@ -107,6 +107,13 @@ void wlr_fbox_transform(struct wlr_fbox *dest, const struct wlr_fbox *box,
 
 #ifdef WLR_USE_UNSTABLE
 
+/**
+ * Checks whether two boxes intersect.
+ *
+ * Returns false if either box is empty.
+ */
+bool wlr_box_intersects(const struct wlr_box *a, const struct wlr_box *b);
+
 /**
  * Returns true if the two boxes are equal, false otherwise.
  */