8d41371457
between two screen coordinates. The screen coordinates are "projected" into isometric space and the vector between them is calculated (except we don't actually do the projection because we can tell just by looking at the points what the projected orientation will be). git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@1257 542714f4-19e9-0310-aa3c-eee0fc999fb1