Smooth auto-scrolling

This is a pure vanity MR that makes auto-scrolling nicer.

Auto-scrolling happens when you drag an object off the edge of the canvas; the canvas scrolls to bring the object into view. I've never thought it worked particularly well; it stutters and zooms off too easily. So I made it smoother and nicer. Here's the before and after:

autoscroll

Note: This took a lot of tweaking of arbitrary parameters to get right. It's possible it'll need tweaking some more.

Anyway, I hope to be able to sneak this in before the feature freeze, as it's a feature I've always wanted. Draft status until I've reviewed all the existing uses of scroll_to_point() to make sure the new auto-scrolling reproduces the same behaviour. Should be fine now.