USD: mark the exporter as Experimental feature

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

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

USD: set Linux-specific #define only on Linux

USD: patch USD to allow Blender to construct a path to JSON files

USD requires some JSON files, and it uses a static constructor to
determine the possible filesystem paths to find those files. This made
it impossible for Blender to pass a path to the USD library at runtime,
as the constructor would run before Blender's main() function. We have
patched USD (see usd.diff) to avoid that particular static constructor,
and have an initialisation function instead.

USD: remove `PXR_PLUGINPATH_NAME` when testing

USD: install libusd_m.a from usd.cmake instead of from harvesting step

USD: Avoid potential conflict with 2nd USD library

We now build the USD library with
`-DPXR_SET_INTERNAL_NAMESPACE=usdBlender`, which replaces the default
namespace `pxrInternal_v_x_y` with `usdBlender`. This is a preventative
measure that avoids possible conflicts when add-ons try to load another
USD library into the same process space.

USD: added note about upstream pull request for PXR_BUILD_USD_TOOLS

USD: no more fatal message in FindUSD.cmake

Instead of breaking the build, it properly sets `USD_FOUND` to `FALSE`.

USD: removed the #include "BLI_iterator.h" from DEG_depsgraph_query.h

This change has been committed to master.

USD: Store BKE_main_blendfile_path(bmain) in a variable

Depsgraph: add missing #include to DEG_depsgraph_query.h

The `DEG_depsgraph_query.h` file uses the `ITER_BEGIN` and `ITER_END`
macros defined in `BLI_iterator.h` without including that header.

No functional changes.

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.