Updates to support core USD release 20.08#617

Merged

kxl-adsk merged 3 commits intoAutodesk:devfrom mattyjams:pr/use_HdEngine_accessor_in_AL_Engine

Jul 28, 2020

+27-7

Commits on Jul 22, 2020

use UsdImagingGLEngine engine accessor for USDIMAGINGGL_API_VERSION 6 and up
mattyjams
committed
pass interpolateMissingClipValues=True when stitching clips in USD versions after 20.05
mattyjams
committed
update build.md to reflect support for core USD release 20.08
mattyjams
committed