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.