Alembic: C++11 doesn't need the space between '> >'

C++11 doesn't need the space between '> >' in a nested templated
declaration, so instead of `std::vector<std::pair<a, b> >` we can now
write `std::vector<std::pair<a, b> >`.

Alembic export: write curve/NURBS as mesh

It's now possible to export curves and NURBS as mesh data to Alembic.
This allows artists to do any crazy thing on curves and export the
visual result to Alembic for interoperability with other software (or
caching for later use, etc.). It's an often-requested feature.

This works around T60503 and the fixes export part of T51311.

Note that exporting zero-width curves is currently not supported, as
exporting a faceless mesh (e.g. just edges and vertices) is not
supported by the mesh writer at all.

To test, create a curve with thickness (for example extruded), export to
Alembic and check the 'Curves to Mesh' checkbox in the export options.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D4213

Alembic export: separated "writing Alembic mesh" from "writing mesh object"

I moved most of the `AbcMeshWriter` code to a new class
`AbcGenericMeshWriter`. The latter is an abstract class and does not
make any assumptions about the type of Blender object being written.
This makes it possible to write metaballs, curves, nurbs surfaces, etc.
as mesh to Alembic files.

The `AbcMeshWriter` class now is the concrete implementation of
`AbcGenericMeshWriter` for writing mesh objects.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D4213

Alembic export: fixed memory leak

If the triangulated mesh was in itself a new mesh that should be freed this
should happen before the function returns (as it only returns a single mesh,
and thus the caller can only free one).

Marked TODOs with the actual string TODO

There are some things that need to be done in Alembic for Blender 2.8 to
make all the 'visible/renderable/selected only' options work.

Alembic: removed declaration of non-existing functions

Removed AbcMeshWriter::getMeshInfo() and AbcMeshWriter::getMaterialIndices().

Properly clean up Python when exiting due to --python-exit-code

When BPY_python_end() is not called, there can be buffered data still in
`sys.stdout` or `sys.stderr`. This generally isn't an issue when those are
connected to a terminal, but when they are read by another process (in the case
of rendering with Flamenco, for example) we could miss the actual error message
that's causing the exit in the first place.

The following script demonstrates the issue; before this commit neither the
writes to STDERR and STDOUT nor the traceback of the NameError were shown.

#!/bin/bash

cat > file-with-errors.py <<EOT
import sys
print('THIS IS STDERR', file=sys.stderr)
print('THIS IS STDOUT', file=sys.stdout)
nonexisting.monkey = 3
EOT

blender --enable-autoexec -noaudio --background
any-existing-blendfile.blend
--python-exit-code 42
--python file-with-errors.py 2>&1 | cat

Reviewers: campbellbarton, mont29

Reviewed By: campbellbarton, mont29

Subscribers: fsiddi

Differential Revision: https://developer.blender.org/D4168

Fix T57078: Alembic curve import: better check for topology similarity

The old code assumed that if the number of curves was the same, the
entire set of curves would have the same topology (in other words, it
assumed 'same number of curves => same number of vertices for each
curve').

I've added a more thorough check that also considers the number of
vertices in each curve. This still keeps certain assumptions in place
(for example that if the topology is the same, the weights won't change,
which is not necessarily true). However, when the assumption doesn't
hold, at least now Blender doesn't crash any more.

Fix T59164: Run Alembic in background when INVOKE'd

This is similar to what physics baking is doing: invoking the operator
runs a background job, whereas executing blocks. This makes Python
scripts calling the Alembic import/export operators more predictable.

For backwardward compatibility with existing Python code the
`as_background_job` parameter still exists, which overrides the
behaviour chosen by INVOKE/EXECUTE.

Reviewers: brecht

Reviewed by: brecht

Differential revision: https://developer.blender.org/D4137/new/

Alembic import: lock interface while importing

This prevents crashes caused by the viewport evaluating the depsgraph while
the import is still running.