graphics: More precise enlarging

Before this change, enlarging a 1px image with x4095 scale was not
accurate and edges were missing. This change fixes this issue.

Updates #611

image_test.go

@@ -961,12 +961,15 @@ loop:
 		src.ReplacePixels(pix)
 
 		_, dh := dst.Size()
-		for i := 32; i < dh; i += 32 {
+		for i := 0; i < dh; {
 			dst.Clear()
 			op := &DrawImageOptions{}
 			op.GeoM.Scale(1, float64(i)/float64(h))
 			dst.DrawImage(src.SubImage(image.Rect(0, 0, w, h)).(*Image), op)
-			for j := -2; j <= 2; j++ {
+			for j := -1; j <= 1; j++ {
+				if i+j < 0 {
+					continue
+				}
 				got := dst.At(0, i+j).(color.RGBA)
 				want := color.RGBA{}
 				if j < 0 {
@@ -977,6 +980,14 @@ loop:
 					continue loop
 				}
 			}
+			switch i % 32 {
+			case 31, 0:
+				i++
+			case 1:
+				i += 32 - 2
+			default:
+				panic("not reached")
+			}
 		}
 	}
 }

mipmap.go

@@ -161,8 +161,8 @@ func (m *mipmap) mipmapLevel(geom *GeoM, width, height int, filter driver.Filter
 	}
 
 	// Use 'negative' mipmap to render edges correctly (#611, #907).
-	// It looks like 256 is the enlargement factor that causes edge missings to pass the test TestImageStretch.
-	const tooBigScale = 256
+	// It looks like 128 is the enlargement factor that causes edge missings to pass the test TestImageStretch.
+	const tooBigScale = 128
 	if sx, sy := geomScaleSize(geom); sx >= tooBigScale || sy >= tooBigScale {
 		// If the filter is not nearest, the target needs to be rendered with gradiation. Don't use mipmaps.
 		if filter != driver.FilterNearest {