USD: renamed `should_export_object` to `should_visit_object`

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

USD: Avoid crash when USD JSON files cannot be found

USD ships with a set of JSON files that define which formats can be written
and which plugins are required. This is used even when statically building
and only using the default USDA and USDC formats.

USD: Fixed static library linking on Linux

This may have inadvertently broken things on Windows, I'll work on a better
way soon.

USD: added explanation of 'USD registry'

USD: reuse `pxr::TfToken` objects

TfToken objects are not cheap to construct, so we do it once and reuse them.

USD: replace check_is_transform_animated() with BKE_object_moves_in_time()

USD: typedef for std::map<short, pxr::VtIntArray>

USD: use BKE_object_visibility()

USD: Clarified some comments, distinguish Hydra from USDViewer

USD: removed dead code

USD: added TODO about checking depsgraph for time dependency

USD: use `BKE_animdata_id_is_animated()`

`BKE_animdata_id_is_animated()` is now used to determine whether object data
has any animation. This also includes counting the fcurves+drivers; previously
the presense of a non-NULL AnimData pointer was enough to consider the
object data animated.

USD: clarification of USDAbstractWriter::is_supported()

USD: iterate() ? iterate_and_write()

USD: 1024 ? FILE_MAX

USD: avoid term 'original' in non-CoW context

Instead of 'original object' the code now mentions 'source' or
'duplisource'.

USD: renamed `prune_the_weak` ? `remove_weak_subtrees` + arguments

USD: rename `link` to `dupli_object` and made variable loop-local

I added an explicit `!= nullptr` to make the formatter put the three parts
of the `for`-statement on three different lines (instead of two), which
increases readability.

USD: made `bool weak_export` `const`