[sdlsound] Sound_Seek() framework in place.

[Ryan C. Gordon]

> playsound.diff adds a --skip option, which is what I used for testing
> this stuff so perhaps it'll be useful to others as well.

I'm actually ripping up playsound to make the command line stuff more
powerful (so arguments like --rate work on a per-file basis, --loop takes
a number for the amount of times to loop, etc). One of the things I've
implemented is a --seek, like this:

   playsound --seek "0:15;8:22;2:30" myfile.wav

...which would seek 15 seconds into myfile.wav, play it for 8 minutes and
22 seconds, and then seek to the 2 minute, 30 second mark and play until
the end of the file.

You can get really fancy and put a --loop in there, too. If this keeps up,
playsound is going to become sentient take over the world.

The rest of the patches I'll look at in the morning, and I'll make sure I
haven't broken everything horribly before committing them.

As always, thanks for taking care of this.

--ryan.