sdl2: Fix broken display updating after the window is hidden

With SDL 2.0.6, calling SDL_ShowWindow during SDL_WINDOWEVENT_HIDDEN
blocks all subsequent display updates.

Instead of trying to override the change, just update the scon->hidden
flag.
Signed-off-by: NJindrich Makovicka <makovick@gmail.com>
Message-Id: <20171112193032.9724-2-makovick@gmail.com>

This is a partial revert of d3f3a0f4,
which in turn is a workaround for a SDL bug.  The bug is fixed in 2.0.6,
see https://bugzilla.libsdl.org/show_bug.cgi?id=3410Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/sdl2.c

@@ -581,14 +581,10 @@ static void handle_windowevent(SDL_Event *ev)
         }
         break;
     case SDL_WINDOWEVENT_SHOWN:
-        if (scon->hidden) {
-            SDL_HideWindow(scon->real_window);
-        }
+        scon->hidden = false;
         break;
     case SDL_WINDOWEVENT_HIDDEN:
-        if (!scon->hidden) {
-            SDL_ShowWindow(scon->real_window);
-        }
+        scon->hidden = true;
         break;
     }
 }