coordinates, or not, depending on the arity of that function. This
enables one to write a simple function from color -> color and
immediately map it on an image. For the situation in which the
function is location-dependent, one can still write a function from
x,y,color -> color and map this on the image.
Added test cases for these to map-image-bsl-tests.rkt.
Modified an error message from colorize, and its test cases.
Added map-image/extra and build-image/extra functions.
