3e3dd1b840
useful delays can be built into the beginning or end of the animation. Detect bogus parameters in the constructor and throw an exception to save developer head-scratching time when the animation doesn't actually do any fading. git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3560 542714f4-19e9-0310-aa3c-eee0fc999fb1