[racket-dev] Icon issues resolved (almost)

I've pushed a change that removes DrRacket's dependence on 
`slideshow/pict' (via `icons'), removes the `icons' module, and adds a 
new `images' collection. Instead of precompiling SVGs, the new code 
ray-traces floating-point ARGB+Z bitmaps; instead of composing icons 
from picts, it composites floating-point renders.

Though the code itself no longer requires picts, I didn't update 
meta/check-dists.rkt or meta/dist-specs.rkt. Doing so looked 
*deceptively* simple, with comments marking the icons hack and 
everything. Eli or Matthew, should I change those files, or let one of 
you do it?

Neil T