Merge topic 'expose-pathtraced-backplate'

85526c12 Fix a parsing bug in RayTracer MaterialLibrary MTL parsing.
30b603e5 Change the default behavior back to what it was before.
aab03198 merge backplate and environment setup methods to avoid duplication
0893f34b Better names and fill in doxygen notes.
c08262b0 let pathtracers use either backplate or environmental light or both

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Michael Migliore <michael.migliore@kitware.com>
Merge-request: !6439