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

Fixed crash when adding/removing custom normals from pinned mesh

When a mesh is pinned in the properties panel, Blender crashes when you
click the "Add Custom Split Normals Data".

The code calls `ob = ED_object_context(C)` which returns NULL when the
mesh is pinned in the properties panel, causing a segfault when trying
to get the mesh via `ob->data`.

A new function `ED_mesh_context(C)` avoids this by first checking
whether a mesh was pinned in the context. If not, it checks the pinned
object's data. If that's not there, or it's not a mesh, it returns the
active object's mesh. Finally it returns NULL if there is no active
object, or if the active object is not a mesh object.

Reviewed By: brecht, mont29

Differential Revision: https://developer.blender.org/D5223

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).