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.
  */

util/box.c

@@ -102,6 +102,15 @@ bool wlr_box_contains_box(const struct wlr_box *bigger, const struct wlr_box *sm
 		smaller->y + smaller->height <= bigger->y + bigger->height;
 }
 
+bool wlr_box_intersects(const struct wlr_box *a, const struct wlr_box *b) {
+	if (wlr_box_empty(a) || wlr_box_empty(b)) {
+		return false;
+	}
+
+	return a->x < b->x + b->width && b->x < a->x + a->width &&
+		a->y < b->y + b->height && b->y < a->y + a->height;
+}
+
 void wlr_box_transform(struct wlr_box *dest, const struct wlr_box *box,
 		enum wl_output_transform transform, int width, int height) {
 	struct wlr_box src = {0};