1.6.21.6

Fixes

Cycles : Fixed rendering of shaders written with IECOREUSD_WRITE_CONFORMANT_OSL_SHADERS=1.
MergeMeshes, MergeCurves, MergePoints : Fixed crash when merge is sorted by a primitive variable and none of the locations to merge contain primitives.
SceneReader : Fixed loading of float-precision orientations from the USD PointInstancer.orientationsf attribute. Previously only the half-precision orientations attribute was supported.