FlxSpriteUtil.alphaMask() should use FlxGraphicAsset

[Gama11]

Currently uses Dynamic and implements the type checking itself instead of using FlxAssets.resolveBitmapData() .

[buckle2000]

We can declare FlxGraphicAsset as abtract type of OneOfThree<...>, and then add @:to toBitmapData or something. So we can do implicit cast on the go.
Is that practicable?

[Gama11]

Not sure why the @:to toBitmapData would be necessary here?

[buckle2000]

Never mind. I think it would just be a more natural way, since if someone want to convert FlxGraphicAsset to BitmapData, one may never check FlxAssets.