sdl: Reverse support for video mode setting

Currently, If the setting of video mode failed, qemu will exit. It
should go back to the previous setting if the new screen resolution
failed. This patch fixes LP#1216368, add support to revert to existing
surface for the failure of video mode setting.
Reported-by: NSascha Krissler <sascha@srlabs.de>
Signed-off-by: NLei Li <lilei@linux.vnet.ibm.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 1378285636-7091-1-git-send-email-lilei@linux.vnet.ibm.com
Signed-off-by: NAnthony Liguori <aliguori@amazon.com>

ui/sdl.c

@@ -86,6 +86,7 @@ static void sdl_update(DisplayChangeListener *dcl,
 static void do_sdl_resize(int width, int height, int bpp)
 {
     int flags;
+    SDL_Surface *tmp_screen;
 
     //    printf("resizing to %d %d\n", w, h);
 
@@ -98,12 +99,26 @@ static void do_sdl_resize(int width, int height, int bpp)
     if (gui_noframe)
         flags |= SDL_NOFRAME;
 
-    real_screen = SDL_SetVideoMode(width, height, bpp, flags);
+    tmp_screen = SDL_SetVideoMode(width, height, bpp, flags);
     if (!real_screen) {
-	fprintf(stderr, "Could not open SDL display (%dx%dx%d): %s\n", width, 
-		height, bpp, SDL_GetError());
+        if (!tmp_screen) {
+            fprintf(stderr, "Could not open SDL display (%dx%dx%d): %s\n",
+                    width, height, bpp, SDL_GetError());
             exit(1);
         }
+    } else {
+        /*
+         * Revert to the previous video mode if the change of resizing or
+         * resolution failed.
+         */
+        if (!tmp_screen) {
+            fprintf(stderr, "Failed to set SDL display (%dx%dx%d): %s\n",
+                    width, height, bpp, SDL_GetError());
+            return;
+        }
+    }
+
+    real_screen = tmp_screen;
 }
 
 static void sdl_switch(DisplayChangeListener *dcl,