SVHS and CVBS improvements

[Stefan-Olt]

Implemented a CVBS automatic gain adjustment / clamping:

• It will take the sync and the black level after time-base-correction as reference
• It will calculate the median of all blank and sync levels of all lines (except top/bottom) to calculate the gain for the entire field
• The gain is averaged over time
• It will clamp based an on the median of the blank level of the line and 2 last and 2 next lines (to compensate for dropouts)
• The sync/blank levels of the last field are used for sync detection of the next field
• Added options: Enable this functionality, set gain multiplier, set fixed gain, set gain average speed

Improved S-VHS sub de-emphasis

• Added second linear de-emphasis
• Adjusted linear & non-linear sub de-emphasis values to match the spec
• Eyeballed de-emphasis values to reduce phase errors as much as possible

Filter-plot:

• Removed old SVHS-specific experiment
• Added reference values to plot (only S-VHS at the moment, but designed for other formats to be added, currently incorrect as only non-linear de-emphasis is plotted)
• Added QSplitter to make image and plot division adjustable with mouse

[Stefan-Olt]

I tested on another sample, SVHS sync problems can dramatically increase, therefore I'm closing the pull request