[client] x11: Find and use the correct valuators for mouse axes

Using the first two valuators present in the event is incorrect.  Events
with only one valuator set, such as those sent by the Xorg evdev driver
when the mouse moved along one axis only, were being discarded.  On the
other hand, mice with multiple scroll wheels may be able to emit events
with two scroll wheel valuators set.

The XInput2 specification is light on details, but "Rel X" and "Rel Y"
appear to be the de facto standard names for the motion valuators.  If
valuators with those labels are not found, fall back to using valuators
with numbers 0 and 1.

client/displayservers/X11/x11.h

@@ -38,6 +38,8 @@ struct X11DSState
 
   int pointerDev;
   int keyboardDev;
+  int xValuator;
+  int yValuator;
 
   bool pointerGrabbed;
   bool keyboardGrabbed;