Blender Git Loki

deps: upgrade USD 20.05 ? 21.02

USD version 21.02 includes two of the changes Blender used to patch in,
which have now been removed from `usd.diff`. Unfortunately 21.02
introduces another issue where LZ4 symbols are accidentally exported,
causing linker errors. Fortunately these symbols are only used for
testing, so I added a patch hunk that simply removes their `extern "C"`
declaration.

The LZ4 linker issue has been reported upstream at
https://github.com/PixarAnimationStudios/USD/issues/1447.