focus-follows-mouse: ignore events generated when reshaping the stage

* Export meta_display_add_ignored_crossing_serial()
* Add the serial for reshaping the stage
* Increase the size of the "ignored_serials" array a bit to
  try to avoid the possibility of losing serials from multiple
  reshapes happening close together.

https://bugzilla.gnome.org/show_bug.cgi?id=597190

src/core/display-private.h

@@ -73,9 +73,11 @@ typedef enum {
 /* This is basically a bogus number, just has to be large enough
  * to handle the expected case of the alt+tab operation, where
  * we want to ignore serials from UnmapNotify on the tab popup,
- * and the LeaveNotify/EnterNotify from the pointer ungrab
+ * and the LeaveNotify/EnterNotify from the pointer ungrab. It
+ * also has to be big enough to hold ignored serials from the point
+ * where we reshape the stage to the point where we get events back.
  */
-#define N_IGNORED_SERIALS           4
+#define N_IGNORED_SERIALS           10
 
 typedef enum {
   META_TILE_NONE,