graphics: Bug fix: Don't touch pixelsToSet when disposing a subimage

This change also removes finalizer for subimages since this is not
necessary.

Fixes #823

image.go

@@ -505,9 +505,7 @@ func (i *Image) SubImage(r image.Rectangle) image.Image {
 	} else {
 		img.original = i
 	}
-
 	img.addr = img
-	runtime.SetFinalizer(img, (*Image).Dispose)
 
 	r = r.Intersect(img.Bounds())
 	// Need to check Empty explicitly. See the standard image package implementations.
@@ -637,8 +635,8 @@ func (i *Image) Dispose() error {
 	}
 	if !i.isSubimage() {
 		i.mipmap.dispose()
+		i.resolvePixelsToSet(false)
 	}
-	i.resolvePixelsToSet(false)
 	runtime.SetFinalizer(i, nil)
 	return nil
 }

image_test.go

@@ -23,6 +23,7 @@ import (
 	_ "image/png"
 	"math"
 	"os"
+	"runtime"
 	"testing"
 
 	. "github.com/hajimehoshi/ebiten"
@@ -1595,3 +1596,24 @@ func TestImageDrawTrianglesWithSubImage(t *testing.T) {
 		}
 	}
 }
+
+// Issue #823
+func TestImageAtAfterDisposingSubImage(t *testing.T) {
+	img, _ := NewImage(16, 16, FilterDefault)
+	img.Set(0, 0, color.White)
+	img.SubImage(image.Rect(0, 0, 16, 16))
+	runtime.GC()
+	got := img.At(0, 0)
+	want := color.RGBA{0xff, 0xff, 0xff, 0xff}
+	if got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+
+	img.Set(0, 1, color.White)
+	sub := img.SubImage(image.Rect(0, 0, 16, 16)).(*Image)
+	sub.Dispose()
+	got = img.At(0, 1)
+	if got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}