CVBS improvements, supergauss, debug_plots, sub-deemphasis, S-VHS, filter-plot 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

vhs-decode:

• Two new debug_plots: luma rf filter and magnitude/density over frequency histogram (as proposed by novgorod)
• Implemeted supergauss (as proposed by novgorod) for luma rf filtering to reduce ringing
• Added option for additional (linear) custom video filters to apply to luma signal
• Added custom filter for S-VHS based sub-deemphasis as shown in service manual
• Tuned sub-demphasis values for S-VHS
• Added option for logistic scaling of sub-deemphasis amplitude

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, can optionally include custom filters and main deemphasis)
• Added QSplitter to make image and plot division adjustable with mouse

[Stefan-Olt]

What is not working correctly is the installation: The two txt files with the filter tables for S-VHS custom filter is not copied by pipx. Not sure if they should be in another directory, but should be sorted out when merging this changes, but I have no idea how pipx works.

[oyvindln]

Yeah I can sort that afterwards, has to be set up in pyproject.toml