sdl: Don't release input on mouse mode change in full-screen mode

While in full-screen mode, the input focus naturally belongs to the SDL
window. Avoid dropping it when switching from absolute to relative
mouse mode.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

ui/sdl.c

@@ -491,7 +491,9 @@ static void sdl_mouse_mode_change(Notifier *notify, void *data)
             absolute_enabled = 1;
         }
     } else if (absolute_enabled) {
-        sdl_grab_end();
+        if (!gui_fullscreen) {
+            sdl_grab_end();
+        }
         absolute_enabled = 0;
     }
 }