Run OBS directly on recording thread

[philpax]

By default, libobs-rs spins up an intermediate thread to run libobs on. @sshcrack recently added support for disabling this intermediate thread by turning off the enable_runtime feature, and suggested that we try it out as we're already using a dedicated recording thread.

This may help tighten timing on things (as one layer of indirection will be removed), but it may also make the blast radius of a crash worse. Let's experiment a bit and see what the impact is.

[philpax]

I spot that @Clyde013 tried this in 7bb6f29, but I don't know what the status of that is

[philpax]

Taking this out of this milestone, too risky