start-time selectable implementations. Modified back buffer to use
BufferCapabilities.FlipContents.COPIED which should do what we need in
terms of providing us with the current frame's contents on which to paint.
git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@2468 542714f4-19e9-0310-aa3c-eee0fc999fb1