box: add wlr_fbox

Same as wlr_box, but for floating-point numbers.

include/wlr/types/wlr_box.h

@@ -18,6 +18,11 @@ struct wlr_box {
 	int width, height;
 };
 
+struct wlr_fbox {
+	double x, y;
+	double width, height;
+};
+
 void wlr_box_closest_point(const struct wlr_box *box, double x, double y,
 	double *dest_x, double *dest_y);