Invalid result of difference() for integral coordinates.

[awulkiew]

For polygons:

green = "POLYGON((-8042 -1485,-8042 250,-8042 250,15943 254,15943 -1485,-8042 -1485))",
blue = "POLYGON((-7901 -1485,-7901 529,-7901 529, 15802 544, 15802 -1485, -7901 -1485))"

The result of difference(green, blue) and/or difference(blue, green) is invalid if the polygons use int coordinate type.

I think it may be related to get_turns/cart_intersect and ratios/fractions but it needs further investigation. I noticed that turns has wrong coordinates, always endpoints of segments are picked as if internally ratios were always truncated to 0.

[mkaravel]

The problem was in the computation of the intersection points. In particular, integer overflow was taking place, yielding the wrong results.

The problem has been fixed by locally promoting the coordinates to a number type with increased precision, thus avoiding the overflow.

See also tickets #10835 and #10658.