[sdlsound] alt audio converter

[Ryan C. Gordon]

> I converted the basic data format of the adapters to Uint8*, so
> hopefully everything except the rate converter is now working. length
> denotes the length of the buffer in Uint8, this seems to be more
> convenient. In the next step I'll address the rate converters.

Frank,

I'm applying this to CVS, but it looks like filter_templates.h got updated
too (since we're missing _varRateUp* and _varRateDown* in the new source).

Please send that so altcvt will compile again.

--ryan.