dix: Allocate MD's motion history to be large enough for potential valuators

Since we can't predict how many valuators may be in a future SD attached to an
MD, we need to preallocate a history buffer that is large enough to keep
MAX_VALUATORS coordinates per event.

In addition, the history buffer needs to memorize the coordinate ranges at the
time, thus requiring MDs to store (min_val, max_val, current_val, time)
instead of (current_val, time) for each motion history entry.

This commit only fixes the allocation.

include/inputstr.h

@@ -167,9 +167,9 @@ typedef struct _ValuatorClassRec {
     int		 	  numMotionEvents;
     int                   first_motion;
     int                   last_motion;
-    void                  *motion; /* motion history buffer */
-
-    WindowPtr    	  motionHintWindow;
+    void                  *motion; /* motion history buffer. Different layout
+                                      for MDs and SDs!*/
+    WindowPtr             motionHintWindow;
 
     AxisInfoPtr 	  axes;
     unsigned short	  numAxes;