Add new dot where cutter crosses stroke - GP

Drawing in Grease Pencil is more similar to vector based as opposed to bitmap, which means that when we erase a dot at the start of a straight segment, the whole line disappears. I can see how it would be hard to change the behaviour of the eraser, but it seems reasonable with the cutter.

When drawing a cutting selection, have a new dot (vertex) added to the stroke where the selection line and the stroke cross each other. Maybe they would need to be a limit radius to check if there's already a dot close enough.