-/* so the idea is this:
- Before processing any events for a given y, store the list+tree of the current active
- list (that might be useful anyway for adding xrow points to segments).
- Then, for every starting segment and for every segment that receives a point, add a (x,segment)
- tuple to a list which we later sort.
- These segments will at that point have their *old* fill styles set.
- Then, order that list, and also merge identical x values using left<->right comparisons on
- the *old* active list, so that for every x coordinate we have the rightmost (old) segment. (For
- small active lists, it might be faster to just mark the segments (and store the x in them), and
- then walk the active list from left to right, collecting marked segments)
- This list now gives us the information about what the fill areas look like above the scanline and to
- the right of segments which received a point.
- Now, apply fill style changes and resort the list, this time using the *new* active
- list for merging identical x values. That gives us the information on what fill areas look like
- *below* the scanline and to the right of segments which received a point.
- Every time an "above" fillstyle differs from a "below" fillstyle, we need to draw a horizontal
- line (from right to left, using the below fillstyle).
-*/
-