The callback-based identification was introduced before job types were added in
7b60529517. The job type should be a more predictable/sane way to identify jobs
that should be exclusive. Using anything else is confusing and non-obvious from
the API usage side. In fact it really confused me when working on #123027.
Checked all existing jobs to make sure behavior is unchanged. Found
two issues:
- `WM_JOB_TYPE_OBJECT_SIM_FLUID` is used for both
`fluid_bake_startjob()` and `fluid_free_startjob()`. It makes sense to
me that they would be exclusive though, so leaving it this way
(meaning they are exclusive now).
- Alembic and USD job types were reused, split them up now to not change
behavior.
Pull Request: https://projects.blender.org/blender/blender/pulls/123033
6887dea786