[sdlsound] alt audio converter

[Ryan C. Gordon]

>
> It comes to my eye, that playsound has the same functionality in the
> callback function, so the padSilence function seems to be redundant. I
> am less than sure of the semantics of buffer, buffer.size, len_mult
> and len_cvt. If it doesn't work, just remove the padSilence function
> and the associated parameters (dst_size, zero, mask). Ryan, may I ask
> you to take a look at this section of the code, too?

The application only sees the converted buffer; the converter should not
try to blank out the end of the buffer if the conversion shrinks the
sound data.

--ryan.