Encode alpha data for lossy with alpha images

[tristanphease]

Currently when encoding alpha data for lossy images it doesn't encode it which causes those images to be very strange looking. This PR just encodes the raw alpha data in an alpha chunk as in https://developers.google.com/speed/webp/docs/riff_container#alpha
Should add the lossless compression method encoding in a later PR, may want to refactor out the lossless encoding code for that.

[fintelia]

I believe that the existing lossless ColorType::L8 could should work pretty well for alpha chunks. Technically it wastes a few bits making sure that the other channels have the right values, but the difference should be very minor.