Fix T54326: Import Alembic stuck in edit mode

Switch to object mode before doing the import.

Merge branch 'master' into blender2.8

Cycles volume: fast empty space optimization by generating a tight mesh
around the volume.

We generate a tight mesh around the active voxels of the volume in order
to effectively skip empty space, and start volume ray marching as close
to interesting volume data as possible. See code comments for details on
how the mesh generation algorithm works.

This gives up to 2x speedups in some scenes.

Reviewed by: brecht, dingto

Reviewers: #cycles

Subscribers: lvxejay, jtheninja, brecht

Differential Revision: https://developer.blender.org/D3038

Merge branch 'master' into blender2.8

Conflicts:
source/blender/blenkernel/BKE_blender_version.h

Smoke: expose empty space clipping property to the UI.

This is used to determine which voxels are to be considered empty space.

Previously it was hardcoded for converting dense grids to OpenVDB grids
to reduce disk space usage.

This value is also useful for rendering engines to know, i.e. to
optimize ray marching.

OpenVDB : use underscores instead of spaces in grid names.

Some other software cannot handle grid names with spaces in them. We still check for names with spaces so as to not break old
files.

This fixes T53802.

Alembic: addition of a simple logging class.

The idea is to have a system where we properly log error messages and
let the users know that errors occured redirecting them to the console
for explanations. This is only implemented for the exporter since the
importer already has similar functionalities; however they shall
ultimately be unified in some way.

Reviewers: sybren, dfelinto

Differential Revision: https://developer.blender.org/D2541

Alembic: addition of a scope timer to perform basic profiling.

Alembic: avoid crashing when reading non-indexed UV params.

Fix T50757: Alembic, assign imported materials to the object data
instead of to the object itself.