ebiten: Add (*Image).RGBA64At (#1773)

Closes #1769.

image.go

@@ -708,6 +708,22 @@ func (i *Image) At(x, y int) color.Color {
 	return color.RGBA{pix[0], pix[1], pix[2], pix[3]}
 }
 
+// RGBA64At implements image.RGBA64Image's RGBA64At.
+//
+// RGBA64At loads pixels from GPU to system memory if necessary, which means
+// that RGBA64At can be slow.
+//
+// RGBA64At always returns a transparent color if the image is disposed.
+//
+// Note that an important logic should not rely on values returned by RGBA64At,
+// since the returned values can include very slight differences between some machines.
+//
+// RGBA64At can't be called outside the main loop (ebiten.Run's updating function) starts.
+func (i *Image) RGBA64At(x, y int) color.RGBA64 {
+	r, g, b, a := i.At(x, y).(color.RGBA).RGBA()
+	return color.RGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}
+}
+
 // Set sets the color at (x, y).
 //
 // Set loads pixels from GPU to system memory if necessary, which means that Set can be slow.

image_test.go

@@ -1582,18 +1582,29 @@ func TestImageAtAfterDisposingSubImage(t *testing.T) {
 	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}
+	want64 := color.RGBA64{0xffff, 0xffff, 0xffff, 0xffff}
+	got := img.At(0, 0)
 	if got != want {
-		t.Errorf("got: %v, want: %v", got, want)
+		t.Errorf("At(0,0) got: %v, want: %v", got, want)
+	}
+	got = img.RGBA64At(0, 0)
+	if got != want64 {
+		t.Errorf("RGBA64At(0,0) 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)
+		t.Errorf("At(0,1) got: %v, want: %v", got, want64)
+	}
+	got = img.RGBA64At(0, 1)
+	if got != want64 {
+		t.Errorf("RGBA64At(0,1) got: %v, want: %v", got, want64)
 	}
 }