Issue with misaligned canvas when editors move relative to top of document.

Hi Jamie, thanks for Mergely, I'm finding it to be a great utility. I came across an issue where the canvases would become misaligned if the CodeMirror editors were repositioned relative to the top of the document. I discovered this  in working on a project involving toggling on and off a "full-screen" view of a file comparison. It can be solved by recalculating the draw_top_offset property after the editors have been repositioned. You'll probably have a better way to solve this, but here's a patch as a starting point for thought. This approach at least satisfies my needs; it does the recalculation when resize is invoked (but not auto-resize, since I always know when the editors are moved and I can call resize at that time).