There ought to be some way of catching this mistake, which is passing
a non-length-1 vector to the swank functions, which then vectorise and
confuse the event stream.

use new fancy :popup-buffer event in slime-media.

Should refactor (and provide width/height options)

Define a generic function to be used with the new
makeReplResultFunction hook, with an implementation using png files
for trellis objects.