jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
The existing code for deriving pixmap settings is pretty complex, brittle and generally bad. Can you explain the changes you're making here? Also, can you explain why we need to use the "easy scale" parameter? I'm REALLY concerned about taming complexity in this part of the code because we've had so many issues with it. Ideally it seems like all the code paths should derive scale settings in the same way.