Check for CC clock signal in ld-process-vbi

[atsampson]

The closed captions on From Dusk Till Dawn (9474 AS) side 3 don't decode very well:

The problem here is that this disc only has CC line 21 on the first field of each frame. ld-process-vbi does some basic sanity-checking, but if the top line of the active area looks enough like a CC signal - e.g. in the shot above - then it'll attempt to decode it, resulting in junk in the CC bytes.

At present we don't check that the CC clock signal is present at the start of the each line. That would be a useful improvement here - it'd filter out nearly all of the spurious lines.

(Edit: unintentional but appropriate issue number for a subtitling problem!)