DSi-based themes: Rework boxart clearing and missing boxarts

[DieGo367]

What's changed?

• Reworked how boxart is cleared to help alleviate some issues. Fixes Residual box art on skins without photo #2033 and UI elements in DSi/3DS theme disappear upon completed loading when positioned next to cover arts horizontally #2220.
  • Things should work™ as long as boxart does not directly overlap other skin UI elements.
• Fixed an issue where the black space around a smaller photo would flicker when color debanding was enabled.
• As suggested in Box-Art Reimplementation #2060:
  • Removed the "?" unknown boxarts in favor of not displaying anything for missing boxart.
  • Changed the default boxart setting to Non-cached.

Where have you tested it?

melonDS 0.9.5
DSi with Unlaunch

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Pull Request status

• This PR has been tested using the latest version of devkitARM and libnds.