Alembic: Fix T52579: crash when replacing slightly different alembic files

Apparently with Maya in a certain configuration, it's possible to have an
Alembic object without schema in the Alembic file. This is now handled
properly, instead of crashing on a null pointer.

Alembic: Fix T52579: crash when replacing slightly different alembic files

Apparently with Maya in a certain configuration, it's possible to have an
Alembic object without schema in the Alembic file. This is now handled
properly, instead of crashing on a null pointer.

Fix T46329: scene_update_{pre,post} doc needs clarification

The documentation for the bpy.app.handlers.scene_update_{pre,post}
handlers states that they're called "on updating the scenes data".
However, they're called even when the data hasn't changed. Of course
such handlers are useful, but the documentation should reflect the
current behaviour.

Reviewers: mont29, sergey

Subscribers: Blendify

Maniphest Tasks: T46329

Differential Revision: https://developer.blender.org/D1535

Fixed Alembic unit test

Commit b6d7cdd3cee9312156e20783248a3b12420b7a53 changed how the mesh data
is deformed, which wasn't taken into account yet in this unit test.

Instead of directly reading the mesh vertices (which aren't animated any
more), we convert the modified mesh to a new one, and inspect those
vertices instead.

Fix T52240: Alembic Not Transferring Materials Per Frame

When a mesh changes its number of vertices during the animation,
Blender rebuilds the DerivedMesh, after which the materials weren't
applied any more (causing the default to the first material slot).

Alembic: Renamed variable assigned_name ? assigned_mat

The variable is a pointer to a Material, not to a name/string.

Alembic import: fix crash when face color index is out of bounds.

This can happen with Alembic files exported from Maya. I'm unsure as to the
root cause, but at least this fixes the crash itself.

Thanks to @looch for reporting this with a test file. The test file has to
remain confidential, though, so it's on my workstation only.

Alembic import: report object name in face color index out of bounds error

Merge branch 'master' into blender2.8

# Conflicts:
# source/blender/makesrna/intern/rna_render.c

Alembic import: report object name in face color index out of bounds error