Fixed an infinite recursive loop between RasterImage and LooksTheSame.
Javadoc
Catch the special case that the transform is the identity, in which case
we don't really need a LinearImage at all.  This shouldn't make a
difference to behavior, but there's a test case (reflecting an image
twice in the y dimension) that this seems to have fixed.  I don't know
why.
Add "Animation" subclass of "World", which automates a lot of common
coding patterns by making the model more explicit.