Add tune option for flat(like cartoon) content

[Andrea Agnolin]

While x264 provide an "animation" tune option, the only content specific tune option in x265 is "grain" which does not work well for flat conent.

x265 should provide a tune option for flat content like cartoon, anime, presentation ...

[Pradeep Ramachandran]

Thanks for the suggestion. I agree that this will be be a useful addition. Patches are welcome :-).

[Blanc]

I second this as Animated content also benefits a lot from this, but on some occasions the lines dissapear and too much smoothing on essential parts appear.

I would certainly like a Anime preset for this as this would help tons.