[sdlsound] can't stop coding

[Ryan C. Gordon]

> if we blank the remaining three quater so that the callback function
> get's a whole buffer and didn't
> has to perform the length check and in the failing case the blanking
> itself, every time it got invoken.

The callback generally expects to write to every byte they are presented;
what they choose to do with it is their own perogative; most applications
always write to every byte, the ones that don't can blank it out
themselves; it's better to leave that to the developer, since we're
talking about writing to several kilobytes of memory that a well-behaved
application will initialize themselves.

> Despite of this I fixed a couple of issues regarding the converter. It
> performs nicely in mono mode.
> Stereo remains untested.

Committed to CVS; thanks!

(I've also patched the regular conversion routines (might as well
call them "stdcvt"), and SDL_sound.c to handle that new parameter to
Sound_BuildAudioCVT()...)

--ryan.