Correct scePsmfPlayerUpdate() finished timing.

With or without audio, it always sets the end on the second call after the
last frame.  Makes sense.
Small correction to psmfplayer seeking.

Should be the specific position after the fileoffset, and should be offset
by the start pts thing...
Fix error codes in scePsmfPlayerGetVideoData().
scePsmfPlayerGetVideoData() keeps going at end.

Even if the video has ended, this happily returns the last frame (with the
same pts value) over and over.  Audio or no audio.

It only returns no data when it's called too early.
Add a class to create helper threads from HLE.

This should be usable from sceIo as well.
Properly schedule psmfplayer video finish.

Fixes #2251, hurray.
Fix errors and scheduling in psmfplayer stop.
Merge pull request #6125 from unknownbrackets/psmfplayer

Fix some minor issues in psmfplayer, schedule finish correctly