RFR: 8358820: Allow interpolation outside of range [0,1]

[Michael Strauß]

JavaFX unnecessarily restricts interpolation in the following ways:
1. `Interpolatable` implementations often clamp intermediate values to the interpolation factor range [0,1].
2. `SplineInterpolator` doesn't accept Y coordinates outside of [0,1] for its control points. While this was probably done so that the computed interpolation factor doesn't exceed [0,1], the restriction goes far beyond that. For example, the following function is not representable, even though its values are all within the [0,1] range:<br>
   <img src="https://github.com/user-attachments/assets/368b6142-052d-4ead-8a59-cbddf4a19660" width="400"/><br>
   The following function is also not representable, but would be very useful for [bouncy animations](https://easings.net/#easeOutBack):<br>
   <img src="https://github.com/user-attachments/assets/af02b044-ae4c-4250-b181-72178ad9f3f3" width="400"/>

Fortunately, there is no technical reason why JavaFX can't support the full range of animations that can be represented with a cubic Beziér interpolation function.

This PR includes the following changes:
1. The specification of `Interpolatable` is changed to require implementations to accept interpolation factors outside of [0,1].
2. All implementations of `Interpolatable` now correctly return intermediate values outside of [0,1].
3. `SplineInterpolator` now accepts control points with Y any coordinate.

-------------

Commit messages:
 - Allow interpolation outside of range [0,1]

Changes: https://git.openjdk.org/jfx/pull/1822/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1822&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358820
  Stats: 785 lines in 52 files changed: 536 ins; 33 del; 216 mod
  Patch: https://git.openjdk.org/jfx/pull/1822.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1822/head:pull/1822

PR: https://git.openjdk.org/jfx/pull/1822