Updates to get this plugin building against Pixar's 18.09 release of USD

See merge request !1

object and call NSIBegin on it. The return value indicates whether or not
the 3Delight library loaded successfully.

_DEBUG. This gives finer grain control over this particular debugging
option, keepin git separate from whether or not USD is being built in
debug mode.

1) Use the NSI::DynamicAPI from HdNSIRenderDelegate. 2) Updated everywhere to use shared NSI::Context. #8

  - new lighting: choice of white, sky or HDRI file
  - fixed headlight xform
  - refactored camera
  - fixed wrong camera xform at init
  - added bunch of new config settings
  - new defaults for pixel samples
  - added env var for winding order
  - fixed several small things