Use the existing PcmDither library.

Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.

This code is unable to dither.  Until we implement that, let's remove
the code for now.  i386 isn't relevant anymore anyway.

16 bit is not enough for volume calculations.

All callers need this shift, so let's move it to the basic method.

Prepare for adding state.

Rename pcm_convert_domain to pcm_domain.  Move it out so we can use it
without depending on the whole PcmConvert library.

Use the SampleTraits template and let the compiler generate a special
case for S32 instead of reusing S24_P32.

Move from PcmClamp().

Allow 32 bit platforms to use 32 bit instead of 64 bit for summing 24
bit samples.

This is really just a mono mapper, but the important part is that this
library cannot fail anymore.