normalize rotate values

A -90 or 630 degree rotation is the same as 270.

Author: willnorris

transform.go

@@ -208,7 +208,8 @@ func transformImage(m image.Image, opt Options) image.Image {
 	}
 
 	// rotate
-	switch opt.Rotate {
+	rotate := float64(opt.Rotate) - math.Floor(float64(opt.Rotate)/360)*360
+	switch rotate {
 	case 90:
 		m = imaging.Rotate90(m)
 	case 180:

transform_test.go

@@ -165,9 +165,12 @@ func TestTransformImage(t *testing.T) {
 
 		// rotations
 		{ref, Options{Rotate: 45}, ref}, // invalid rotation is a noop
+		{ref, Options{Rotate: 360}, ref},
 		{ref, Options{Rotate: 90}, newImage(2, 2, green, yellow, red, blue)},
 		{ref, Options{Rotate: 180}, newImage(2, 2, yellow, blue, green, red)},
 		{ref, Options{Rotate: 270}, newImage(2, 2, blue, red, yellow, green)},
+		{ref, Options{Rotate: 630}, newImage(2, 2, blue, red, yellow, green)},
+		{ref, Options{Rotate: -90}, newImage(2, 2, blue, red, yellow, green)},
 
 		// flips
 		{