Add background color as an appearance setting

[radther]

What

Adds a new background color option to the appearance pane of settings. This color should fill the screens background, replacing the existing black/clear appearance.

Why

Allows for more customisation theming without needing background images. The color can be combined with the other colors for fun themes.

Main page	Quick Menu	Appearance Settings

How

1. Un-commenting the existing setting
2. Replacing uses of clearing background layer with fill rect of the new background color (Color 7)
3. Update rendering of certain elements (clock and button hint pills) to render with this color behind to fix anti-aliasing
4. Reorder rendering to render BG layer behind the selection pill layer

[frysee]

Gave it a brief test, a couple of issues I found:

• On boot, the initial screen will not render any background image unless you navigate once.
• Opening and closing quick menu will also lead to the same behavior.
• Game switcher is just black?
• When scrolling, I'm seeing a brief flickering that I feel wasnt there before.

[frysee]

We could just do a CFG_getColor(COLOR_BACKGROUND) here, and avoid having to maintain and call PLAT_setBackgroundColor at all.

[frysee]

Gave it a brief test, a couple of issues I found:

1 On boot, the initial screen will not render any background image unless you navigate once.
2 Opening and closing quick menu will also lead to the same behavior.
3 Game switcher is just black?
4 When scrolling, I'm seeing a brief flickering that I feel wasnt there before.

1: Still happening
2: Seems fixed
3: Game switcher still sometimes stays black, and sometimes it renders - but with the screenshot overlaid over the pills and texts.
4: Still there.