Blender Git Loki

USD: disabled debug print

No functional changes.

USD: Export graph now is indexed by (export parent, duplicator)

By including the duplicator into the export graph, each duplicated object
now has a unique entry in the graph. This solves a problem with the
transform matrices.

Merge remote-tracking branch 'origin/master' into sybren-usd

USD: only write timeSamples when we write animated data

By using the default timecode USD won't even write a single `timeSample`
for non-animated data. Instead, it writes it as non-timesampled. This makes
the USD file more explicit of our intent, and allows for read-time
optimisations.

USD: be more selective about what to export each frame

To prevent too large USD files, we now have a bit of a framework to only
write data on the first frame for non-animated objects. This needs more
work, though, as it can be made more selective.

USD: Initial support for exporting animated data

Mesh and transform animation are now written when passing `animation=True`
to the export operator. There is no inspection of whether an object is
actually animated or not, so this will likely export too much information
(each mesh is written for each frame in the scene).

The administration of which timecode to use for the export is left to the
file-format-specific concrete subclasses of `AbstractHierarchyIterator`;
the abstract iterator itself doesn't know anything about the passage of
time.

USD: Commented-out most debug prints

Things are starting to work, so the debug prints aren't necessary any more.
They will be completely removed at some point, but I'm keeping them in the
code for now so that I can quickly enable them when necessary.

No functional changes.

BKE_anim: added comment to explain the matrix `DupliObject.mat`

`DupliObject.mat` contains the world matrix of the duplicated object, and
this knowledge is now available in a comment.

USD: Introducing experimental USD support

This introduces very limited USD support:

- The USD libraries are expected to be in /opt/usd/, not yet built by
install_deps.sh.
- The code requires cleanup & some restructuring to simplify once the
way we want to iterate over the depsgraph is deemed ok.
- No support for instancing, animation, or other features; all
duplicated objects are made real in the USD file. This is fine for
exporting a linked-in posed character, not so much for thousands of
pebbles etc.
- Debug prints are still in place.

USD: Avoid use of `default` in `switch`

By avoiding `default` and listing all valid `OB_xxx` cases explicitly the
compiler can warn us about any missing cases in the future.

Include BLI_iterator.h in DEG_depsgraph_query.h

The `DEG_xxx_ITER_yyy` iterators need the `ITER_BEGIN` and `ITER_END`
macros defined in BLI_iterator.h.

USD: Iterate depsgraph, rather than view layer bases

The depsgraph contains all info we need, and iterating it is more
future-proof than iterating the view layer. This also removes any use of
original data and makes the USD exporter work 100% on evaluated data.

Also contains some other cleanups.

USD: Pass 'selected/visible only' flags to the exporter

This isn't handled properly by the exporter yet.

USD: replaced 'xform_only' with 'weak_export'

An object marked as `weak_export` will only be exported if it has any non-
weak children. When it is exported, it is only written as transform, and
not as full object (so no object data).

USD: removed ExportSettings class

The `ExportSettings` class was a leftover from the porting of Alembic code,
and is no longer necessary. Without it the code is easier to follow.

No functional changes.