Make PLAYMODE_ONCE streams respect loop points.

[SiegeLord]

The current behavior appears to be undefined. We could define it as follows: take the loop start and end to define a slice of the stream to play, and stop playing when the stream position reaches the loop end.

Relevant code: https://github.com/liballeg/allegro5/blob/master/addons/audio/kcm_mixer.c#L217

It might be as easy as changing that piece of code to respect loop_start/end, but need to verify:

• That we don't define this behavior already
• How the proposed behavior interacts with seeking/stopping/resetting loop points.

Alternatively, maybe add a "sub stream" API where we create a new stream with new starts and ends.