Support multi-planar video capture stream

[timbz]

I have a device that only supports V4L2_CAP_VIDEO_CAPTURE_MPLANE (doesn't support V4L2_CAP_VIDEO_CAPTURE). Would you be interested in a PR that adds support for multi-planar API?

The current diff shows the changed needed to make fswebcam work on my device. Obviously it needs to be properly integrated to support both types of buffers depending on the capability.

I could add enum v4l2_buf_type buf_type to src_v4l2_t that gets set in src_v4l2_get_capability and add if (s->buff_type == ...) blocks around the relevant code parts.