Blender Git Loki

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

USD: reduce hair colours to one colour per hair strand

The colours weren't written correctly, so this just reduces the amount of
data that's written. This'll probably be used until we decide to evaluate
the hair material.

USD: initial support for exporting lights

USD does not seem to support spot lights, so those aren't exported yet.
The units used for the light intensity is also still a bit of a mystery.

USD: prevent crash when writing hair system with empty pathcache

Not sure what causes `psys->pathcache` to be `nullptr` in the first place,
but at least this prevents Blender from crashing.

USD: Experimental support for instancing/referencing

Dupli-object meshes are now written to USD as references to the origina
mesh. This is still very limited in correctness, as there are issues
referencing to materials from a referenced mesh.

I am still committing this, as it also introduces some code clarifications
and gives us a place to start when continuing the quest for proper
instancing in USD.

USD: Fix missing writer pointer

Fixes bug introduced in previous commit.

USD: removed 'parent_writer' pointer

The `parent_writer` pointer was intended to be used by an Alembic writer,
but now it's getting in the way. I'm removing it from the
`AbstractHierarchyIterator` with the intent of either restoring it or
finding another solution when we actually implement Alembic writers.

USD: added debug function for printing the export graph

USD: Changed ExportGraph type to hold HierarchyContext pointers

This allows us to change the stored HierarchyContext objects in a future
commit.

No functional changes.

USD: Cleanup, removed commented-out debug code

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

USD: Fixed issue with mesh particle export

When mesh particles were exported, the object name was still used for each
instance of that object. As such, only one instance was written to USD.
This is now fixed by suffixing the object name with the particle's
persistent ID, giving each particle XForm a unique name.

Particles are still only written when they are alive, which means that they
are always visible (there is currently no code that deals with marking them
as invisible outside their lifespan).

USD: Fixed bug getting evaluation mode property

USD: Make USD_PATH configurable via a CMake variable

The default is still `USD_PATH=/opt/usd`, and using USD still requires you
to manually install USD without Python bindings.

USD: added USD export to the File > Export menu

This also required exposing the build option `bpy.app.build_options.usd`.

USD: Avoid writing invisible particle emitter meshes

USD: Offer choice for viewport or render settings

The exporter operator now takes a parameter to choose the depsgraph
evaluation mode (Viewport/Render). This makes it easier to switch between
exporting particle hair (Render) or mesh hair (Viewport).

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

USD: Export UV maps of meshes

Each UV map is stored on the mesh in a separate primvar. Materials can
refer to these UV maps, but this is not yet exported by Blender.

USD: Write mesh single/doublesidedness

USD seems to support neither per-material nor per-face-group
double-sidedness, so we just use the flag from the first non-empty
material slot. If there is no material we default to double-sidedness.