The height convention for decoding frames with only a single field made sense
for compatibility with legacy decoders, but doesn't really match the convention
used by NDI, which is the primary (only?) user. Thus, change it to simply
assuming that if the two fields overlap, the frame is meant to be a single
field and the frame height matches the field height.
Signed-off-by: James Almer <jamrial@gmail.com>