X11 backend: tell app that mouse focus has been obtained when grabbing the

input. This allows us to enable DGA mode, even when the system cursor was
 outside of the window when the grab was initiated.

Fixes Bugzilla #299, which has a more detailed explanation of the issue.

--HG--
branch : SDL-1.2

src/video/x11/SDL_x11wm.c

@@ -369,6 +369,8 @@ SDL_GrabMode X11_GrabInputNoLock(_THIS, SDL_GrabMode mode)
 
 		/* Make sure we register input focus */
 		SDL_PrivateAppActive(1, SDL_APPINPUTFOCUS);
+		/* Since we grabbed the pointer, we have mouse focus, too. */
+		SDL_PrivateAppActive(1, SDL_APPMOUSEFOCUS);
 	}
 	XSync(SDL_Display, False);