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

[Nir Lisker]

On Tue, 4 Jun 2024 20:46:28 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> @nlisker would you be willing to be the second reviewer?

I could do some of the review, but probably no time for a full one.

> Since you propose interpolating objects that aren't simple sets of floating-point fields, the overridden interpolate method should specify the behavior of the fields that are not. For example, different types of Paint are sometimes interpolatable and sometimes return either the start or end; some fields of BackgroundImage are interpolatable and others (notably the image itself) return either the start or the end; and so forth.

When I did the (simple) interpolation work on [Point2D/3D](https://bugs.openjdk.org/browse/JDK-8226454) I also looked at `Border` and `Background` because animating the color of these is rather common. The problem is that they are rather complex and don't interpolate trivially as a whole. As a user, if I want to interpolate between images, I would think of a smooth transition between them, not a jump cut. There are other oddities as well.

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PR Comment: https://git.openjdk.org/jfx/pull/1471#issuecomment-2151084870