Add default antialiasing option to FlxSprite

[ACrazyTown]

For performance reasons games might include an antialiasing option in their settings menu. But implementing an antialiasing option, especially in a project that's nearing completion is tedious because you'd have to change the antialiasing value for every sprite.

This PR adds a defaultAntialiasing option to FlxSprite, which sets its antialiasing value upon creation to defaultAntialiasing.

So instead of changing every sprite's value manually, you could simply do

FlxSprite.defaultAntialiasing = true;

and it would automatically set antialiasing to true on every sprite upon creation.

[Geokureli]

thanks!