Add low latency mode in nvcuvid/nvdecode

[abrevet]

When I use h264_cuvid, it takes 4 extra frames to pass before it returns the first frame. So, this decoder is not usable for low latency application.
After browsing the NvDecode API, there is a parameter to manage the number of extra frames: ulMaxDisplayDelay

This value is hard-coded in FFmpeg to 4, see libavcodec/cuvid.c line 980:

ctx->cuparseinfo.ulMaxDisplayDelay = 4;

It would be useful to add a 'low latency' option in FFmpeg to set the parameter ulMaxDisplayDelay to zero (no latency).
For example, an equivalent mode already exists when using QuickSync:

av_dict_set(&m_opt, "async_depth", "1", 0);

Thank you

[Hendrik]

The decoder would be terribly slow without this delay, fwiw.

[abrevet]

Replying to heleppkes:

The decoder would be terribly slow without this delay, fwiw.

I tested my application with ulMaxDisplayDelay = 0 (HD definition), and I have not seen low performance.

It would be useful to let the choice to the user.

[Balling]

Really? ​https://github.com/FFmpeg/FFmpeg/blob/cc86982fc5f6ae7a25f448931dd3fc2b80c2ce0b/libavcodec/cuviddec.c#L1015