Add a gloss-raster wrapper to inspect an image without animation.

[Ben Lippmeier]

From Dmitry Dzhus

Not sure if I sound reasonable, but didn't you think about adding a
playWorld version without stepWorld (typed Float -> world -> world)
argument? So that only external events cause the world to change. For
cases when computations are intensive, and the «field» doesn't evolve
by itself, but only reacts to user events (like changing camera
position in raytracer), isn't it wasteful to redraw a world if no
events occured?