v4l2-compliance: reworked the (c)v4l-helpers.h headers

v4l2-compliance: reworked the (c)v4l-helpers.h headers

These headers were designed to easy coding for the V4L2 API, in particular
hiding the complexity relating to single and multiplanar formats.

These reworked headers resolve a number of design issues and they are now ready
to be used elsewhere.

As an example, the C++ code below works for single and multiplanar video capture and
output, vbi capture and output and sdr capture. By replacing V4L2_MEMORY_MMAP with
_USERPTR it will work with userptr streaming as well. But note that the code is
missing error handling, so a real application will need to do a bit more.

int main(int argc, char **argv)
{
	cv4l_fd fd;
	unsigned cnt = 0;

	fd.open(argc >= 2 ? argv[1] : "/dev/video0");

	cv4l_queue q(fd.g_type(), V4L2_MEMORY_MMAP);

	q.reqbufs(&fd, 4);
	q.obtain_bufs(&fd);
	q.queue_all(&fd);
	fd.streamon();

	cv4l_buffer buf(q);

	while (fd.dqbuf(buf) == 0) {
		printf("seqnr: %u bytesused[0]: %u bytesused[1]: %u\n", buf.g_sequence(),
				buf.g_bytesused(0), buf.g_bytesused(1));
		fd.qbuf(buf);
		if (cnt++ >= 60)
			break;
	}
	q.free(&fd);
	fd.close();
}

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>