As previously discussed, the CrystalHD hardware treats some PAFF
clips different from others; even when input fields are always in
separate packets, the hardware might return a single fieldpair for
one clip and individual fields for another.

Given the bogus flags set by the hardware, it is impossible to
distinguish these two cases without knowing about the current
picture and the next one. The hardware can usually provide the
picture number of the next picture and when that is available,
we can detect the two cases.

When it is not available, we have to guess - and find out later
if we were right or wrong.

With this change, clips will play correctly unless they are PAFF
where individual fields are returned *and* no next picture number
is available. Generally speaking, the incorrect cases arise in
the first couple of seconds of a clip as the delay calibration takes
place. Once that's set, things work fine.