[client] x11: change to a more visible "dot" cursor

This change alters the small square dot cursor to a more visible 16x16
cursor for X11. A new option `spice:largeDotCursor` can be set to use an
alternative 32x32 cursor for the vision impaired.

client/src/main.h

@@ -218,6 +218,7 @@ struct AppParams
   bool                 autoCapture;
   bool                 captureInputOnly;
   bool                 showCursorDot;
+  bool                 largeCursorDot;
 
   int                  audioPeriodSize;
   int                  audioBufferLatency;