dix: add whiteroot flag

Add a -wr option to use a white root window, and use a BackPixel rather
than BackPixmap for both white and black root windows.

dix/globals.c

@@ -143,6 +143,7 @@ FontPtr defaultFont;   /* not declared in dix.h to avoid including font.h in
 Bool loadableFonts = FALSE;
 CursorPtr rootCursor;
 Bool blackRoot=FALSE;
+Bool whiteRoot=FALSE;
 ClientPtr requestingClient;	/* XXX this should be obsolete now, remove? */
 
 _X_EXPORT TimeStamp currentTime;