Keeping these as linked datablocks to the brush does not match the idea
that assets should generally be appended, and leads to some confusing
situations with linked materials on objects. Now use either a local
material with matching weak library reference or make a local copy if
it does not exist yet.
This also add weak library references to the materials in the 2D Animation
template, so they will be reused.
A problem is that weak library references include a full path to assets
blend files, including the Blender version for the essentials assets files.
This means weak library references do not work across platforms and
Blender versions.
Another known limitation is that if the (linked) Brush Asset material is
edited, and there is already a local copy of it, this local copy will
remain unchanged and will be used by future strokes as well.
Ref #131186
Pull Request: https://projects.blender.org/blender/blender/pulls/134226
c650977534