WindowServer+LibGUI: Shrink window edge resize hot-spots

The hot-spots for resizing a window by dragging its corner are now
limited to a small area around the actual corner instead of an area with
1/3rd the length or width of the window.

The hot-spots to resize a window while holding a modifier key and the
right mouse button are unchanged.

Userland/Services/WindowServer/Compositor.cpp

@@ -426,6 +426,9 @@ void Compositor::compose()
                 backing_rect.set_right_without_resize(window_rect.right());
                 backing_rect.set_top(window_rect.top());
                 break;
+            default:
+                VERIFY_NOT_REACHED();
+                break;
             }
 
             Gfx::IntRect dirty_rect_in_backing_coordinates = rect.intersected(window_rect)