Option to skip or select VBI lines for teletext insertion

[premultiply]

For some applications it would be usefull to have an option to skip (blank - no teletext output) in VBI for teletext insertion. By default all VBI lines from 7 to 22 are filled with teletext at the moment. For example skipping line 16 - 18 would allow to insert VPS (german data line application for VCR control and station id) and VITS test signals. Or reducing the total count of teletext lines would limit the datarate for DVB/DTV encoding and tunneling of teletext.

[ali1234]

This would be a useful feature for coarse speed control too, since fewer lines means subpages stay on screen longer.

There is one small problem though: the software cannot know which field is being displayed because the API does not support it. So to work around this it draws the same data into both fields and updates it every frame. This means you would need to skip lines in both fields.

There is a way to work around this called double weave but it is more complex to implement so I did not bother as it was unnecessary. But given this and other feature requests it might be time to do it.

[Tachiorz]

https://github.com/Tachiorz/raspi-teletext/commit/9b014884116d81b4989f43c5fd2bc90435864937 It isn't pretty, I was lazy to make proper options, but it works

[ZXGuesser]

I forgot that someone had already had a go at this, and did a version where both fields use the same lines. https://github.com/ZXGuesser/raspi-teletext/commit/39a328e81fafa40e626c9f375950854623209133 In mine I kept the command line interface backwards compatible. https://github.com/ZXGuesser/raspi-teletext/commit/3c402fef018c7736173d61dde387022016769fdf

[ZXGuesser]

I've now incorporated @Tachiorz code for separate allocations on each line, but keeping my backwards compatible command arguments (and line blanking masks instead of line allocations) https://github.com/ZXGuesser/raspi-teletext/commit/25bed4e7b37002ecd9629adc30923692658ab2ba

[ali1234]

A version of this got merged, thanks.