Fix: Geometry nodes not depending on instanced collections properly

Geometry nodes were not adding referenced instanced collections as
dependencies to depsgraph.

This would lead to meshes and data not being ready on evaluation in
certain cases.

Make the different weight paint code paths exclusive to each other

Before you could have the new sculpt symmetry code and the older weight
paint symmetry code active at the same time. This would lead to users
easily trashing their weigh paint data if they were not careful when
switching between modes.

Now the specific weight paint symmetry code is an exclusive toggle so
the user can't accidentally mirror strokes and vertex groups at the same
time. This also paves the way of supporting Y and Z symmetry in the
future for weight groups mirroring if we decide to add it in the future.
(As the UI part for it is done)

See https://developer.blender.org/D10426

WIP bake buttons

Buggy and naming and settings needs to change probably.
See TODOs in the code

Cleanup more unused variables and functions

Cleanup various lineart files, add FIXME comments.

Fix build issues.

Merge branch 'master' into temp-lineart-contained

Merge branch 'master' into temp-lineart-contained

Cleanup: Remove LibAV support

Ubuntu and Debian dropped libav in 2015 in favor of ffmpeg.
Development stopped of libav in 2018, so it should be save to remove.

Geometry Nodes: Add Collection Info Node

Implements a node to get collection objects.
These objects are then passed along as instances in the node tree.

Follow up tasks:
Multiple nodes does not support instancing yet: T85159
Changing collection offset does not trigger a refresh: T85274

Reviewed By: Jacques, Dalai, Hans

Differential Revision: http://developer.blender.org/D10151