Since b1526dd2c6, viewport renderer sets `G.is_rendering`, but VSE scene
rendering function used this to decide whether to do offscreen opengl
render or use render API. This logic was quite weak.
Use `SeqRenderData::for_render` instead of `G.is_rendering`, since it
explicitly defines whether strip rendering is invoked by F12 render job.
Since offscreen gl rendering rely on `BLI_thread_is_main()` being
true, use this to initialize `do_seq_gl` variable for clarity.
The use of render job path was further conditioned on `G.is_rendering`,
with exception of running headless, but this condition was incorrect.
This condition was reformulated as precondition, which if true, returns
`nullptr` instead of crashing.
Pull Request: https://projects.blender.org/blender/blender/pulls/115079
f4e28d4762