> D3 doesn’t get reset because it’s guaranteed to be 0 at the beginning of each scanline
There's no such guarantee. The NEXTHOR loop only inverts for points which are to the absolute left of the point being tested ("IS HORIZ <= PT.H ? \\ NO, IGNORE THIS POINT").
Imagine that, for every point, there's a line of inversion that goes all the way down to the bottom of the bounding box. For a typical rectangular region, there's going to be four inversion points - one for each corner of the rectangle. The ones on the bottom cancel out the ones on the top. To add a second disjoint rectangle to the region, you'd simply include its four points as well; so long as the regions don't actually overlap, there's no need to keep track of whether they share any scan lines.
There's no such guarantee. The NEXTHOR loop only inverts for points which are to the absolute left of the point being tested ("IS HORIZ <= PT.H ? \\ NO, IGNORE THIS POINT").
Imagine that, for every point, there's a line of inversion that goes all the way down to the bottom of the bounding box. For a typical rectangular region, there's going to be four inversion points - one for each corner of the rectangle. The ones on the bottom cancel out the ones on the top. To add a second disjoint rectangle to the region, you'd simply include its four points as well; so long as the regions don't actually overlap, there's no need to keep track of whether they share any scan lines.