text_renderer: sapi: fix and simplify code, and rework as a dedicated COM worker

The module need to live in a MTA COM thread, which cannot really be guaranteed when run from the video output thread or transcode thread (via spu_Render()) and the module cannot really report this kind of errors and was aborting.

Instead, create a new dedicated thread that will always have MTA mode enabled, and synchronize commands with it.

Missing: a few comments in commits need to be improved