dix: more device scaling insanity.

Assuming master->last.valuators is in screen coords, SD's are always in device
coordinates.

1. If an event comes in, scale masters->last to the device, drop into device's
last->valuators.
2. Apply motion from the actual event
3. Scale back to screen coords, check if we may need to cross screens
4. Drop screen coords into master->last
5. Rescale to device coords, drop into deviceValuator event and SD->last
6. Drop screen coords into ev->root_x/y

Whoopee...

include/inputstr.h

@@ -422,6 +422,8 @@ typedef struct _DeviceIntRec {
     } u;
 
     /* last valuator values recorded, not posted to client;
+     * for slave devices, valuators is in device coordinates
+     * for master devices, valuators is in screen coordinates
      * see dix/getevents.c */
     struct {
         int             valuators[MAX_VALUATORS];