[client] input: avoid reentrancy when realigning guest cursor

This prevents LGMP_ERR_QUEUE_FULL from happening with high polling rate
mice, which is caused by receiving many more mouse events while the
guest cursor warps, triggering more warps.

client/src/main.h

@@ -250,6 +250,9 @@ struct CursorState
   /* true if the guest should be realigned to the host when next drawn */
   bool realign;
 
+  /* true if the guest is currently realigning to the host */
+  bool realigning;
+
   /* true if the cursor needs re-drawing/updating */
   bool redraw;