v4l2bufferpool: set buffer interlace flags when field is V4L2_FIELD_INTERLACED
If v4l2_buffer.field is V4L2_FIELD_INTERLACED, we set corresponding GstVideoBuffer flags depending on the video standard. According to V4L2 specification, M/NTSC transmits the bottom field first, all other standards the top field first. https://bugzilla.gnome.org/show_bug.cgi?id=737603