dix: store the xi2mask on grabs and pass it around as needed.

This enables passive and implicit passive grabs for XI2 events, except that we
don't have the protocol spec yet to request them.

include/inputstr.h

@@ -181,6 +181,8 @@ typedef struct _GrabRec {
     CursorPtr		cursor;		/* always NULL for keyboards */
     Mask		eventMask;
     Mask                deviceMask;     
+    /* XI2 event masks. One per device, each bit is a mask of (1 << type) */
+    unsigned char       xi2mask[EMASKSIZE][XI2MASKSIZE];
 } GrabRec;
 
 typedef struct _KeyClassRec {