acenc: LAME-inspired window decision

This performs quite a bit better than the current 3GPP-inspired window decision
on all the samples I have tested. On the castanets.wav sample it performs very
similar to iTunes window selection, and seems to perform better than Nero.
On fatboy.wav, it seems to perform at least as good as iTunes, if not better.
Nero performs horribly on this sample.

Patch by: Nathan Caldwell <saintdev@gmail.com>

Originally committed as revision 24892 to svn://svn.ffmpeg.org/ffmpeg/trunk