Support ATF atlases in the runtime.

A lot of context can be muddled through in the disucussion here:
https://github.com/threerings/flump/issues/53.

In order to use ATF atlases in your runtime:
 * Unzip your flump archive
 * Modify library.json (or .xml), add a property to the top level element: "textureFormat":"atf"
 * Replace all of the .png atlases in the archive with your .atf encoded atlases
 * Zip it all back up and it should be good to go.

Tested on Mac Destop (Adobe claims Mac and Windows desktops are the same. It does not mention
Linux) and iPad 2. Some notes for people that might want to dive down this path:
 * One of Adobe's docs currently claims that transparency is not supported in the block-based
   compression formats, but this is incorrect, it works just fine.
 * My flump project is unusual (comosed primarily of parts of photos that were originally JPEGs, but
   I was still unable to get the quality of ATF textures up high enough to be usable in the end
   product. This is a great shame because the loading time was definitely better, and the flump
   archive was up to 70% smaller, if I built for iOS only. Traditional flump projects (composed of
   vector art from Flash symbols) will probably be even harder to get looking good using ATF.
 ** To this end I tried both with and without the -c flag to png2atf. When not using the -c flag, I
   tried -q 0: this both looked worse and was larger than my .png flump comparison. I also fiddled
   a great deal with creating my own PVRTC files both via texturetool (from XCode) and with
   PVRTexTool and then converting them to ATFs. I was unable to get a texturetool PVR to convert
   via ATF, and the PVRTexTool output looked just as bad as what I got via png2atf. I suspect this
   is partly because ATF only seems to accept PVRs that are using 4bits per pixel, so right off
   the bat you're losing half of your color depth.