Reduce copying in fillPolygon

A slice of points means copying every point into the slice, then
copying every point's data from the slice to TrianglesData. An
array of indicies lets the compiler make better choices.

imdraw/imdraw.go

@@ -354,11 +354,12 @@ func (imd *IMDraw) fillPolygon() {
 	imd.tri.SetLen(imd.tri.Len() + 3*(len(points)-2))
 
 	for i, j := 1, off; i+1 < len(points); i, j = i+1, j+3 {
-		for k, p := range []point{points[0], points[i], points[i+1]} {
-			(*imd.tri)[j+k].Position = p.pos
-			(*imd.tri)[j+k].Color = p.col
-			(*imd.tri)[j+k].Picture = p.pic
-			(*imd.tri)[j+k].Intensity = p.in
+		for k, p := range []int{0, i, i + 1} {
+			tri := &(*imd.tri)[j+k]
+			tri.Position = points[p].pos
+			tri.Color = points[p].col
+			tri.Picture = points[p].pic
+			tri.Intensity = points[p].in
 		}
 	}