input: abstract Xtst device lookup

The callers should need to use the dev privates key to look up xtest
devices.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Benjamin Close <Benjamin.Close@clearchain.com>

include/input.h

@@ -497,6 +497,7 @@ extern int AllocXtstDevice(ClientPtr client,
                              char* name,
                              DeviceIntPtr* ptr,
                              DeviceIntPtr* keybd);
+extern BOOL IsXtstDevice(DeviceIntPtr dev, DeviceIntPtr master);
 
 /* misc event helpers */
 extern Mask GetEventFilter(DeviceIntPtr dev, xEvent *event);