If conditions are not met for offscreen rendering, render job is used as
fallback. This can be skipped if there is possibility of race condition.
This was only considered for case where `context->for_render` was false,
since this indicates that call is invoked by the job itself.
If scene strip display is set to `OB_RENDER`, it needs to start the
render job. So variable `do_seq_gl` was renamed to `is_preview` and
reused in race avoidance connditon. This was done to convey clearer
meaning and match comments.
This change does make code less safe - race is avoided by UI not
allowing to run F12 render and opengl render at the same time. However,
I don't think, this could be easily resolved on VSE side without
suspending thread or being able to run multiple render jobs.
Pull Request: https://projects.blender.org/blender/blender/pulls/127926
580b4a1a2d