correction in canvas drawing for editor position

[yofreke]

My page contains a mergely-editor dom element which has a position of {top: 42, left: 0}, causing all of the canvas draws to be 42px too far down. Because of this, I needed to offset the margin indicator draw calls by the position of the editor node. I am not sure if this is the most reasonable solution to the problem, but it seemed like a bug that needed solving.

Let me know if there is a preferred way of doing this which is not in this PR.