Blender Git Loki

USD: Write edge creases to USD mesh

Only the crease is written to USD. The "Sharp" edge flag doesn't seem to
be supported by USD, or at least I couldn't find it in the API docs.

USD: Write current Blender version into USD file

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

USD: Add missing curly braces

No functional changes.

USD: use a struct USDMeshData to group mesh data for USD export

USD Export: stop when user presses escape

HAIR TEST

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

USD: Added exporting the camera

Only perspective cameras are supported for now. I also had to add a function
`BKE_camera_sensor_size_for_render()` that calculates the sensor width
or height, depending on the fitting parameter and the scene render
resolution aspect ratio.

USD: export meters-per-unit scale in stage metadata

When set to a non-metric unit system, Blender internally still stores
imperial units, so the scale is the only thing we need to write to USD.

USD: also export the last frame

USD: improved exporter progress reporting

USD: hair export is now optional

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

USD: Start of hair export support

Only the parent strands are exported, and only with a constant colour.
No UV coordinates, no information about the normals.

USD: abstracted the `ensure_xxx_writer` functions

The `ensure_xform_writer` and `ensure_data_writer` were very similar, and
in preparation of also creating hair and particle system writers I turned
them into one higher-order function that can be used for all those
purposes.

No functional changes.

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

USD: reworked mesh writing code

The geometry and material handling is now done in separate functions, and
has been slightly optimised for clarity.

USD: support for simple preview materials

Very simple versions of the materials are now exported, using only the
viewport diffuse RGB, metallic, and roughness.

When there are multiple materials, the mesh faces are stored as geometry
subset and each material is assigned to the appropriate subset. If there
is only one material this is skipped.

The first material if any) is always applied to the mesh itself
(regardless of the existence of geometry subsets), because the Hydra
viewport doesn't support materials on subsets. See
https://github.com/PixarAnimationStudios/USD/issues/542 for more info.

Note that the geometry subsets are not yet time-sampled, so it will
break when an animated mesh changes topology.

USD: Better detection of whether transforms are animated

When an object in a duplicated collection has a parent that's not in that
collection, it needs to check its entire parent chain for animatedness
(because those parents aren't going to be in the USD hierarchy for that
duplicated object).