PAL - Dropouts outside possible range

[IanSmallshire]

Tested over multiple samples

When plotting the extents of Start / End within a line and also the line numbers I have identified these:

• negative video line numbers for dropouts
• PAL + NTSC max dropout start is after max dropout end (thought they were cut at the end of a line)
• Without negative lines 0-313 is is 314 inclusive which is higher than 0-312 expected (625/2 = 312.5 rounded up for half line)

[happycube]

@oyvindln has a fixed version already, going to merge it into my current working branch and then merge it up:

https://github.com/oyvindln/ld-decode/blob/vhs_decode/vhsdecode/doc.py#L136

[happycube]

I merged above in a while ago, need to check to see if it's fixed.

[happycube]

Making another change to dropout detection - if I got it right, it'll block out any off-field dropouts from being seen at all.

[happycube]

(feel free to reopen if I didn't fix it)