... instead of hard-coding it to a ridiculously high value that
makes bandwidth-starved devices unhappy.

libao (in SVN) does the same thing, and this calculation was indeed
taken from it.

Low-bandwidth USB (1.1) sound devices seem to need this to prevent
underrun / broken pipe errors (during hw setup, no less) from being
triggered.

git-svn-id: https://svn.musicpd.org/mpd/trunk@4362 09075e82-0dd4-0310-85a5-a0d7c8717e4f