Don't change the object's active material when adding a material slot. This
active material will be used by brushes with no pinned material, while the
added slot is meant for the pinned material.
The if-statement in `BKE_grease_pencil_object_material_ensure_from_brush()`
would change the active unpinned material, even though it was only meant to
handle the pinned material. This was not a new issue, it was just unlikely to
run into it.
Before brush assets were introduced this if-statement would not be
executed when activating any of the default brushes. Afterwards it would
be, because the material was newly linked (because the brush asset was
linked) and so there was no material slot created for it yet.
Pull Request: https://projects.blender.org/blender/blender/pulls/132865
4b2a4341a7