Corrected aliasing-related bugs in `Edge#reverse` and `Edge#simplify`.
Initial code for finding filled areas.
First we unshare the edges that they could be used as edges in separate filled shapes.
One edge is forward, the other one is backward, to keep their appropriate fill styles always on their left side.
Then we group them by fill style to prepare for joining their contours into closed fill areas.
(This will be dealt with next time, hopefully.)