Fix upside down and performance issues all in one

[shunia]

It's been a long time since I work with Unity and WebP, thanks for this library and it has helped a lot. And when I finally solved the issues I created, I thought I should give my solution back here. So I organized my code and create the pull request here.

This pr can not be merged, just for the library owners' information.

This pr can:

1. Fix upside down problem: Animated WebP parse error on MacOS Editor and Android #18 (comment)
2. Fix performance issue (10x faster decoding): Pixel copy to flip texture is too slow, WebPDecoderOptions.flip should be used, but how? #21
3. Offer render ability for further extending

And because I'm no longer working on Unity, and Unity just crashes my laptop when opening, this pr can not:

1. Run!!! (I can not make the project run because when I open Unity and my laptop just crashes), so the codes may not able to passed the build;
2. Be used directly: The actual loading logic should be implemented (WebPLoader.cs#32)

[netpyoung]

You found the way. good job and thank you pull request.