Add shell.background-color.rgba to prefs

[wusyong]

With surfman#260, it's possible for all platforms to display transparent background now. This could let users to make more native-like interface as they wish. Right now, servo will still make viewport background white. And I hope it can add a config to toggle this to transparent background.

Based on feedbacks on zulip, this PR adds shell.background-color.rgba to prefs with the resource file prefs.json set to [1.0, 1.0, 1.0, 1.0] by default.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes doesn't fix any github issue.
• These changes do not require tests because this is only a background for shell's viewport.

[mrobinson]

I really think should allow setting a background color (which can be transparent) as that is what all other WebView APIs. do.

[wusyong]

Alright, how about changing it to background_color: { r: f64, g: f64, b: f64, a: f64} (as I didn't see the option to set it as f32 in prefs).

[mrobinson]

Alright, how about changing it to background_color: { r: f64, g: f64, b: f64, a: f64} (as I didn't see the option to set it as f32 in prefs).

That seems sensible. Thanks.

[mrobinson]

Is it possible to represent this as an array like:

"shell.background-color.red": [1.0, 1.0, 1.0, 1.0],

[wusyong]

It'll need to add new variant like serde's Value::Array to PrefValue. I can try to add this.

[wusyong]

@mrobinson I added new variant PrefValue and make sure other crates can handle it well.
It probably could add a macro to handle From trait between array and this vraint.
But I suppose it's better to wait until there's more configs that require array.
So we can implement more ergonomic one.

[mrobinson]

Nice.