Use a saner sanity check for floating_reposition.

The function contained_by_output checks whether any output contains any
parts of a give rect. Rather than relying on the central point of the rect.

src/floating.c

@@ -659,11 +659,7 @@ void drag_pointer(Con *con, const xcb_button_press_event_t *event, xcb_window_t
 void floating_reposition(Con *con, Rect newrect) {
     /* Sanity check: Are the new coordinates on any output? If not, we
      * ignore that request. */
-    Output *output = get_output_containing(
-        newrect.x + (newrect.width / 2),
-        newrect.y + (newrect.height / 2));
-
-    if (!output) {
+    if (!contained_by_output(newrect)) {
         ELOG("No output found at destination coordinates. Not repositioning.\n");
         return;
     }