limit the size of patterns

Creating a pattern the size of the current image is potentially
very dangerous if the user has a, say, 10000x10000 image. If
the image is big (aribtrarily defined as having a dimension > 1000),
the pattern is scaled down to fit in a 1000x1000 box

Note: we really nede someone to pick up maintenance of our pattern
creator, especially since patterns are getting more and more useful.