avcodec/prores_aw : add support for prores 444 with alpha

only 16b alpha is supported (not 8 bits)

following official encoder, alpha data doesn't impact
yuv plane quality.

So the alpha data encoding is done after the yuv part.
It's also avoid to loose quality in yuv part when
alpha is not uniform.

the alpha encoding funcs is mainly take from prores_ks
encoder, except for the alpha data reorganization