IECoreScene CurvesPrimitiveEvaluator : fix vTangent() error

[proberts-cinesite]

Generally describe what this PR will do, and why it is needed.

IECoreScene.CurvesPrimitiveEvaluator.Result.vTangent() now returns vector pointing in the correct (opposite) direction for curves with Linear interpolation.

Breaking Changes

changes (fixes) bevaviour of function.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.

[johnhaddon]

@ivanimanishi, do you have any concerns about this fix from the IE point of view? It's one of those ones that which is a bugfix but affects the output in a way you might consider to be a breaking change if you'd already adapted to getting the wrong result.

[ivanimanishi]

It's difficult to be sure it won't cause issues, but I'm willing to try it out and deal with compatibility issues if necessary later.

[johnhaddon]

I've been through the Gaffer codebase and don't see anything that will be impacted by this fix, so from our point of view we're happy to go ahead in a patch version as well. Paul will update Cinesite's internal code to account for the change when it gets released.

We need to get a release containing #1471 out sharpish, so the plan is to hold back on merging this PR until that is done and then include this in the following release.

[johnhaddon]

@proberts-cinesite, we need to merge this to RB-10.5 rather than main - could you rebase and update the PR please? If you had time to add test coverage in test/IECoreScene/CurvesPrimitiveEvaluatorTest.py that would be even better.