Blender Git Loki

USD: copy JSON files to datafiles/usd

Previously it copied to datafiles/usd/usd.

USD: Store BKE_main_blendfile_path(bmain) in a variable

USD: properly mark unused variable

USD: pass data from invoke() to execute() in a struct

This makes the code somewhat more complex, but more explicit and
future-proof as well.

Cleanup: USD: clarified a comment

USD: Fix exporting of meshes of dupli-objects

The dupliobject's `base_flag` variable needed to be copied from its
duplicator.

USD: clarified comment about defining _GLIBCXX_PERMIT_BACKWARD_HASH

USD: moved all code into C++ namespace 'USD'

USD: renamed 'xform' to 'transform'

Sergey and I don't agree on 'tfm' vs. 'xform', so I just chose to not to
use either and avoid abbreviating altogether.

USD: use LISTBASE_FOREACH

No functional changes.

USD: private indicators

No functional changes.

USD: set Linux-specific #define only on Linux

USD: Report progress on a frame basis and with less guesswork

USD: mark the exporter as Experimental feature

This will allow us to merge the USD exporter into master before it's
completely finished.

FIXES for rebase

USD: enable USD in Release configuration

USD: fixed copyright year in license blocks

USD: replace `#define` with anonymous enum

USD: expanded explanation of `HierarchyContext::weak_export`

USD: also stop job when `*stop = true`