background: fix loading background along with scene

[yookoala]

Why this PR is needed?

To reproduce my issue

1. Create a script with scene and background together, for example:

   monogatari.script ({
   // The game starts here.
   'Start': [
   	'show scene #555555 with fadeIn',
   	'show background phone.jpg with fadeIn',
   	'Hello.',
   	'Time to test saving',
   	'After save',
        ],
   });

2. Run the game.
3. When you see "Hello.", save the game.
4. After save, simply load the newly saved game.

Expected behaviour

See the word "Hello" with the background "phone.jpg".

Actual behaviour

See the word "Hello" with the background color #555555. No phone.jpg.

What this PR does?

• Background should be of higher priority than scene. So the presents
  of Scene should not prevent Background from taking effect.

[yookoala]

I'm not 100% sure about this. The original scene variable check might be doing something essential. But it did fix the problem for me.