Add webp export option alpha_quality

[mzealey]

cwebp has a commandline flag of -alpha_q <x>% which uses lossy-compression for the alpha channel and can significantly reduce image size, and also seems to speed up image conversion in some cases. It would be great to have this ability in Pillow also.

Additionally an option similar to -noalpha where we know the image does not contain any alpha layer saves ~27 bytes per image - not particularly significant but again a nice-to-have for cwebp compatibility.

Thanks!

[radarhere]

noalpha isn't mentioned in https://developers.google.com/speed/webp/docs/api or https://developers.google.com/speed/webp/docs/container-api, or anything similar, so I don't think this is a setting that we can use.

If you want to ensure that your Pillow image does not have transparency, that seems like just a matter of making sure the mode isn't one of "LA", "La", "PA", "RGBA" or "RGBa", making sure "transparency" isn't in the image's info, and making sure that it doesn't have a palette with an alpha channel.

[radarhere]

I've created #7872 to resolve this by adding alpha_quality.

[mzealey]

Oh brilliant thank you this looks great :-)