nvenc support for 10-bit modes with HEVC

[Antoine Martin]

Follow up from #1260 (nvenc 7) and #909 (10-bit colour).

According to ​NVIDIA VIDEO CODEC SDK: HEVC 10-bit encoding (..) require Pascal generation GPUs.

According to the docs:

• NV_ENC_BUFFER_FORMAT_YUV420_10BIT 10 bit Semi-Planar YUV [Y plane followed by interleaved UV plane]. Each pixel of size 2 bytes. Most Significant 10 bits contain pixel data.
• NV_ENC_BUFFER_FORMAT_YUV444_10BIT 10 bit Planar YUV444 [Y plane followed by U and V planes]. Each pixel of size 2 bytes. Most Significant 10 bits contain pixel data.
• NV_ENC_BUFFER_FORMAT_ARGB10 10 bit Packed A2R10G10B10. Each pixel of size 2 bytes. Most Significant 10 bits contain pixel data.
• NV_ENC_BUFFER_FORMAT_ABGR10 10 bit Packed A2B10G10R10. Each pixel of size 2 bytes. Most Significant 10 bits contain pixel data.

The "each pixel of size 2 bytes" looks wrong to me (probably true for YUV formats only): this conflicts with "A2B10G10R10" / "A2R10G10B10". It looks like plain 32-bit data (2 alpha + 3*10 bit) we get from X11, which would be nice.
Another way would be to use nvenc via ffmpeg (ie: ​added support for 10 bit HEVC encoding), meh.

[Antoine Martin]

See also #1347 and #1317