IECoreScene CurvesPrimitiveEvaluator : fix vTangent() error

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

Author: proberts-cinesite

Changes

@@ -7,11 +7,7 @@ Fixes
 - USDScene :
   - Fixed writing of shutter values from cameras without a `shutter` parameter. The `shutter:open` and `shutter:close` attributes are now omitted instead of being written with Cortex's default -0.5, 0.5 shutter values.
   - Fixed reading of shutter values from cameras without `shutter:open` and `shutter:close` attributes. The `shutter` parameter is now omitted instead of being created with USD's default 0, 0 shutter values.
-
-10.5.x.x (relative to 10.5.15.0)
-========
-
-
+- CurvesPrimitiveEvaluator : Fixed error in `Result::vTangent()` function causing tangent to point in opposite direction for curves with `Linear` interpolation.
 
 10.5.15.0 (relative to 10.5.14.1)
 =========

src/IECoreScene/CurvesPrimitiveEvaluator.cpp

@@ -260,8 +260,8 @@ void CurvesPrimitiveEvaluator::Result::init( unsigned curveIndex, float v, const
 	{
 		m_coefficients[0] = 1.0f - m_segmentV;
 		m_coefficients[1] = m_segmentV;
-		m_derivativeCoefficients[0] = 1.0f;
-		m_derivativeCoefficients[1] = -1.0f;
+		m_derivativeCoefficients[0] = -1.0f;
+		m_derivativeCoefficients[1] = 1.0f;
 		m_vertexDataIndices[0] = m_varyingDataIndices[0] = o + i;
 		if( periodic )
 		{