Support converting 32 bit samples to any other supported sample
format.

Added code to convert all other sample formats to 32 bit.

There is nothing 24 bit specific in the pcm_dither_24 struct.  Since
we want to reuse the struct for 32 bit dithering, let's drop the "_24"
suffix from the struct name.

Replace a "static" buffer with the PCM buffer library.

The function does not need the full pcm_convert_state struct.

Moved all code which converts the sample format to a separate library.