Blender Git Loki

USD: Revamped building & bundling process

- USD is now built as monolithic library, instead of 25 smaller
libraries. We were linking all of them as 'whole archive' anyway, so
this doesn't affect the final file size. It does, however, make life
easier with respect to linking order, and handling upstream changes.
- The JSON files required by USD are installed into bin/usd; they are
required on every platform.
- USD is patched so that it finds the aforementioned JSON files in a path
relative to the running executable. When left unpatched, the only way
to influence the search path for those files is to set an environment
variable BEFORE Blender starts (USD uses static initialisers, so by
the time the USD exporter runs, it has already unsuccessfully tried
loading the JSON files, and won't re-investigate that environment
variable ever again).
- USD is patched to have a `PXR_BUILD_USD_TOOLS` CMake option to disable
building the tools in its `bin` directory. This is sent as a pull
request at https://github.com/PixarAnimationStudios/USD/pull/1048