Second part to fix#128420.
On startup, the Blender File Outliner mode would show empty libraries
linked, pointing to the brush essentials files. This was because some
grease pencil versioning code would call
`BKE_paint_ensure_from_paintmode()`, which would link in the default
brushes. Then a bit later, brush assets versioning code would remove
local brushes from the default starup file, so the library link became
empty.
Initializing paint data shouldn't necessarily include importing default
brushes. In an earlier version I made this optional with a boolean, but
it's easy enough to separate out entirely.
Now `BKE_paint_ensure()` just initializes paint data, and
`BKE_paint_brushes_ensure()` has to be called to ensure that active
brushes are available.
Pull Request: https://projects.blender.org/blender/blender/pulls/128801
Julian Eisel
2499299ff2