RFR: 8332895: Support interpolation for backgrounds and borders [v2]

[Michael Strauß]

On Fri, 2 Aug 2024 15:00:38 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> I agree. Not sure if I should remove it from all of the StyleableProperty implementations, as the `origin = null` is already there.
>
> I know what @kevinrushforth would say: "it is better to avoid introducing unrelated changes"
> 
> as long as the new code omits `= null` we should be fine.
> 
> (rationale for omitting: explicit initialization creates an unnecessary step in the debugger)

I've removed the `= null` for consistency on both fields.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1522#discussion_r1702023744