Separate LZW compression from writing for parallel encoding

[kornelski]

Fixes #42

Adds write_lzw_pre_encoded_frame function that takes an already-compressed LZW data. This allows the caller to perform compression separately, e.g. using a thread pool to compress multiple frames in parallel.

The Frame docs say the buffer could be something else, so I'm taking advantage of that to smuggle the LZW data in there.

[HeroicKatora]

Neat. And what a nice coincidence that the documentation already hints at this possibility 😄

[kornelski]

What are your plans for merging and releasing this?

[HeroicKatora]

What are your plans for merging and releasing this?

Soon, but rather as 0.12. The feature change in color quant is not 100% compatible (if default-features = false is being used) and even this seems major enough to require some level of awareness.