graphics: Bug fix: (*Image).At must return color.RGBA type value

image.go

@@ -250,12 +250,12 @@ func (i *Image) ColorModel() color.Model {
 //
 // At loads pixels from GPU to system memory if necessary, which means that At can be slow.
 //
-// At always returns color.Transparend if the image is disposed.
+// At always returns a transparent color if the image is disposed.
 //
 // At can't be called before the main loop (ebiten.Run) starts (as of version 1.4.0-alpha).
 func (i *Image) At(x, y int) color.Color {
 	if i.isDisposed() {
-		return color.Transparent
+		return color.RGBA{}
 	}
 	// TODO: Error should be delayed until flushing. Do not panic here.
 	clr, err := i.restorable.At(x, y)

image_test.go

@@ -378,9 +378,18 @@ func TestImageDispose(t *testing.T) {
 		t.Fatal(err)
 		return
 	}
+	img.Fill(color.White)
 	if err := img.Dispose(); err != nil {
 		t.Errorf("img.Dipose() returns error: %v", err)
 	}
+
+	// The color is transparent (color.RGBA{}).
+	// Note that the value's type must be color.RGBA.
+	got := img.At(0, 0)
+	want := color.RGBA{}
+	if got != want {
+		t.Errorf("img.At(0, 0) got: %v, want: %v", got, want)
+	}
 }
 
 func min(a, b int) int {