x0vncserver: support for forward mouse button

This change adds support for an additional button in x0vncserver,
allowing the 9th bit (forward mouse button) to be used when calling
XDesktop::pointerEvent().

unix/x0vncserver/XDesktop.cxx

@@ -242,9 +242,9 @@ void XDesktop::init(VNCServer* vs)
 void XDesktop::start()
 {
   // Determine actual number of buttons of the X pointer device.
-  unsigned char btnMap[8];
-  int numButtons = XGetPointerMapping(dpy, btnMap, 8);
-  maxButtons = (numButtons > 8) ? 8 : numButtons;
+  unsigned char btnMap[9];
+  int numButtons = XGetPointerMapping(dpy, btnMap, 9);
+  maxButtons = (numButtons > 9) ? 9 : numButtons;
   vlog.info("Enabling %d button%s of X pointer device",
             maxButtons, (maxButtons != 1) ? "s" : "");