sdl: Limit sdl_grab_end in handle_activation to Windows hosts

There are scenarios on Linux with some SDL versions where
handle_activation is continuous invoked with state = SDL_APPINPUTFOCUS
and gain = 0 while we grabbed the input. This causes a ping-pong when we
grab the input after an absolute mouse entered the window.

As this sdl_grab_end was once introduced to work around a Windows-only
issue (0294ffb9), limit it to that platform.

CC: Erik Rull <erik.rull@rdsoftware.de>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

ui/sdl.c

@@ -828,10 +828,14 @@ static void handle_mousebutton(DisplayState *ds, SDL_Event *ev)
 
 static void handle_activation(DisplayState *ds, SDL_Event *ev)
 {
+#ifdef _WIN32
+    /* Disable grab if the window no longer has the focus
+     * (Windows-only workaround) */
     if (gui_grab && ev->active.state == SDL_APPINPUTFOCUS &&
         !ev->active.gain && !gui_fullscreen) {
         sdl_grab_end();
     }
+#endif
     if (!gui_grab && ev->active.gain && is_graphic_console() &&
         (kbd_mouse_is_absolute() || absolute_enabled)) {
         absolute_mouse_grab();