Request to add deblocking param for webp images #1803

I'm producing webp images with Thumbor v6.0.1 https://github.com/thumbor/thumbor/tree/6.0.1which uses Pillow version > 3.0. Normally this works very well but there are some images which webp's deblocking (https://developers.google.com/speed/webp/docs/cwebp#options see -f option) functionality completely ruins an images. Not sure what webp default value for deblocking is but I used cwebp with deblocking set to 0 and the resulting image was perfectly fine.

Source image:

Webp image produced by Thumbor (converted back to PNG for browser compatibility):

Webp image produced by cwebp with deblocking set to zero (converted back to PNG for browser compatibility):

This pattern of webp's deblocking slaughtering "low poly" / "pixel" and other blocky art style images has been observed quite a few times via our Thumbor setup. I can make a PR on Thumbor's side if Pillow can add support for passing an option for deblocking. Other options like -af aka auto-filter would be great too, but deblocking is a top priority for me.

If a code bounty would help motivate, I'd definitely be willing to contribute. I'd be happy to contribute myself but I really don't know where to get started, others would likely be far more productive. However, if that's the only option please point me in the right direction and I'll see what I can do.

Lastly, if there is a way that I could build Pillow via some sort of custom webp library that defaults to a level of 0 deblocking, I could also try that.

Any assistance is GREATLY appreciated. Thank you