ui/sdl2: fix reset scaling binding to be consistent with gtk

Problem: Currently the reset scaling hotkey is inconsistent between SDL
and GTK graphics modes.

Solution: Fix SDL to use MOD+0 instead of MOD+u which is in line with
GTK and generally more consistent with other apps.

This is also related to my previously sent patch fixing the docs.

Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Nir Lichtman <nir@lichtman.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250910114929.GA1783677@lichtman.org>

ui/sdl2.c

@@ -421,7 +421,7 @@ static void handle_keydown(SDL_Event *ev)
                 sdl_grab_end(scon);
             }
             break;
-        case SDL_SCANCODE_U:
+        case SDL_SCANCODE_0:
             sdl2_window_resize(scon);
             if (!scon->opengl) {
                 /* re-create scon->texture */