mpvcore: add a note about desync on track switches
af_volume: remove unused features

Roughly follows MPlayer svn commits 36492 and 36493. We also remove
the volume peak reporting. (There are much better libavfilter filters
for this, I think.)
af_volume: don't change volume if nothing is to be changed

On the float path. Note that this skips clipping, but we expect that
everything on the audio-path is pre-clipped anyway.
af_volume: uncrustify

Also, use more C99 and remove "register" keywords.
af_volume: switch to new option parsing