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

The primary purpose of this pull request is to allow this plugin to build against the latest 18.09 release of USD. In addition, the renderer plugin IsSupported method was implemented by attempting to create an NSI render context so that apps can avoid trying to start this plugin if it isn't going to work. Finally, the use of the debug output driver is now hidden behind an NSI_DEBUG define, rather than using the _DEBUG define. Using _DEBUG made it impossible to build a debug version of USD without also having this plugin send all renderer output to a floating window.