Add GPU acceleration option

[JeromeMartinez]

Preparation of the arrival of GPU accelerated FFV1 encoder/decoder in FFmpeg.

[retokromer]

Decoder or encoder?

[JeromeMartinez]

Decoder or encoder?

Both are expected. But current FFmpeg patch proposal is only encoder for the moment, we are testing and polishing it before checking the decoder part (the developer says that most of the code is common to both so should not be long).

[retokromer]

Indeed!

I was troubled by #448 (comment) mentioning decoder.

[JeromeMartinez]

Oops, edited.

[stephenmcconnachie]

@JeromeMartinez @retokromer is there a firm timescale for GPU encoding (then onward to decoding)? Obviously I am very interested.............

[JeromeMartinez]

@stephenmcconnachie a beta version of the encoder should be there this week. I send you an email.