Experimental support for the NV12 pixel format

[dkogan]

Hi. This patch adds support for NV12. This has some issues, and isn't ready to merge as is.

I don't have genicam hardware that produces this pixel format, but I do have such a v4l2 stream (via v4l2loopback) that this patch enables. Concerns:

• The value of ARV_PIXEL_FORMAT_NV12 is made up. I don't entirely understand where this is supposed to come from, and the 5555 part is arbitrary
• arvv4l2stream.c was assuming that an image is made up of rows with bytes_per_line_data each, and the whole thing has payload_size * bytes_per_line_data bytes. This isn't true for NV12. It has bytes_per_line = width and payload_size = widthheight1.5 and bytes_per_line_data isn't well-defined. Can we simply remove bytes_per_line_data and the logic that uses it?

Thanks