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rasterizer: more precise area calculation

Avoid division on every segment in the loop. Instead do it
only once. (Un)surprisingly, the rendered glyph maps are
different after the change.

--- a/rast.c

+++ b/rast.c

@@ -336,7 +336,7 @@

 qbzr(SegQ *seg, int ns, int dw, int dh, int pitch, u8int *p)

 {

 	int ju², j², w, h, i, j, ju, px, py, v;

-	Sval rju², s₀, s, *c, *c₀x, *c₀y, *cs, zx, zy;

+	Sval rju², s₀, s, sl, sq, *c, *c₀x, *c₀y, *cs, zx, zy;

 	u64int *ma, *mb, all, nall;

 	ju² = closest²(dh);

@@ -352,20 +352,20 @@

 	}

 	/* calculate area */

-	s₀ = 0;

+	sl = sq = 0;

 	for(i = 0; i < ns; i++){

 #define det(a,b) (a.x*b.y - a.y*b.x)

-#define cL(s) det(s.p0, s.p2)/2.0

-#define cQ(s) (2*det(s.p0, s.p1) + 2*det(s.p1, s.p2) + det(s.p0, s.p2))/6.0

+#define cL(s) det(s.p0, s.p2)

+#define cQ(s) (2*det(s.p0, s.p1) + 2*det(s.p1, s.p2) + det(s.p0, s.p2))

 		if(seg[i].p1.x == seg[i].p2.x && seg[i].p1.y == seg[i].p2.y)

-			s₀ -= cL(seg[i]);

+			sl -= cL(seg[i]);

 		else

-			s₀ -= cQ(seg[i]);

+			sq -= cQ(seg[i]);

 #undef det

 #undef cL

 #undef cQ

 	}

-	s₀ *= QBZR_PIX_SCALE;

+	s₀ = (sl + sq/3)*QBZR_PIX_SCALE/2;

 	/* working space:

 	 * quick is-segment-in-the-cell tests (two 64-bit nums per segment)