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: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/ui/sdl.c b/ui/sdl.c
index 385cc91..c7aaedf 100644
--- a/ui/sdl.c
+++ b/ui/sdl.c
@@ -491,7 +491,9 @@
             absolute_enabled = 1;
         }
     } else if (absolute_enabled) {
-        sdl_grab_end();
+        if (!gui_fullscreen) {
+            sdl_grab_end();
+        }
         absolute_enabled = 0;
     }
 }