Use simpler calculation to intersect polygon edges with tile edges.

Add the polygon that produced a bad tile with the previous version
as a test.

geometry.cpp

@@ -641,36 +641,25 @@ static bool inside(draw d, int edge, long long minx, long long miny, long long m
 	exit(EXIT_FAILURE);
 }
 
-// http://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect
-static draw get_line_intersection(draw p0, draw p1, draw p2, draw p3) {
-	double s1_x = p1.x - p0.x;
-	double s1_y = p1.y - p0.y;
-	double s2_x = p3.x - p2.x;
-	double s2_y = p3.y - p2.y;
-
-	double t;
-	// s = (-s1_y * (p0.x - p2.x) + s1_x * (p0.y - p2.y)) / (-s2_x * s1_y + s1_x * s2_y);
-	t = (s2_x * (p0.y - p2.y) - s2_y * (p0.x - p2.x)) / (-s2_x * s1_y + s1_x * s2_y);
-
-	return draw(VT_LINETO, p0.x + (t * s1_x), p0.y + (t * s1_y));
-}
-
 static draw intersect(draw a, draw b, int edge, long long minx, long long miny, long long maxx, long long maxy) {
+	// The casts to double are because the product of coordinates
+	// can overflow a long long if the tile buffer is large.
+
 	switch (edge) {
 	case 0:  // top
-		return get_line_intersection(a, b, draw(VT_MOVETO, minx, miny), draw(VT_MOVETO, maxx, miny));
+		return draw(VT_LINETO, a.x + (double) (b.x - a.x) * (miny - a.y) / (b.y - a.y), miny);
 		break;
 
 	case 1:  // right
-		return get_line_intersection(a, b, draw(VT_MOVETO, maxx, miny), draw(VT_MOVETO, maxx, maxy));
+		return draw(VT_LINETO, maxx, a.y + (double) (b.y - a.y) * (maxx - a.x) / (b.x - a.x));
 		break;
 
 	case 2:  // bottom
-		return get_line_intersection(a, b, draw(VT_MOVETO, maxx, maxy), draw(VT_MOVETO, minx, maxy));
+		return draw(VT_LINETO, a.x + (double) (b.x - a.x) * (maxy - a.y) / (b.y - a.y), maxy);
 		break;
 
 	case 3:  // left
-		return get_line_intersection(a, b, draw(VT_MOVETO, minx, maxy), draw(VT_MOVETO, minx, miny));
+		return draw(VT_LINETO, minx, a.y + (double) (b.y - a.y) * (minx - a.x) / (b.x - a.x));
 		break;
 	}