Added first and last fieldNumber to the videoParameters metadata

[simoninns]

Added some psuedo videoParameters to the LdDecodeMetaData class to provide the first and last active frame/field numbers. This is set automatcally according to the isSourcePal flag.

The idea is to only have these constants in one place which I think will help towards issue #401

[simoninns]

Let me know if I missed any... I think I found all of the instances.

[atsampson]

This looks sensible as a starting point. A couple of suggestions:

• Add a comment describing the semantics of these variables in the LdDecodeMetaData header: "These are half-open ranges, where lines are numbered sequentially from 0 within each field or interlaced frame." Some parts of the code, e.g. the display in ld-analyse and SourceVideo::getVideoField, currently use 1-based sequential field/frame line numbers and/or closed ranges... and the standard PAL/NTSC numbering system uses 1-based numbers with the fields not interleaved, which is different again.

• Copy over the comments from comb.h and palcolour.h saying where these numbers came from and how they map to the standard PAL/NTSC line numbers (the "Interlaced line ..." ones).