Xi: reset motion history when swapping device classes.

This isn't really the correct solution, but it'll have to do until I figured
out how to transfer the history over correctly.

Xi/exevents.c

@@ -574,6 +574,8 @@ DeepCopyDeviceClasses(DeviceIntPtr from, DeviceIntPtr to)
         memcpy(v, from->valuator, sizeof(ValuatorClassRec));
         v->motion = NULL;
         AllocateMotionHistory(to); /*XXX should be copied somehow */
+        v->first_motion = 0;
+        v->last_motion = 0;
 
         v->axes = (AxisInfoPtr)&v[1];
         memcpy(v->axes, from->valuator->axes, v->numAxes * sizeof(AxisInfo));