af_volume: Avoid pointlessly calculating the maximum volume.

Only calculate it when it will be used, and do not calculate
it per-channel.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36497 b3059339-0415-0410-9bf9-f77b7e298cf2
af_volume: optimize for common case where all channels
have same volume.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36498 b3059339-0415-0410-9bf9-f77b7e298cf2