Option to auto-adjust the level of 0 IRE based on the back porch

[Stefan-Olt]

This introduces the new option --ire0_adjust, which will calculate the value of 0 IRE like this:

• It's all done in the hz_to_output function
• The position of the back porch is set as an fixed attribute in the FieldNTSCShared and FieldPALShared classes
• Calculate the median value of the back porch in each line (median is a better choice than mean, if there is for example a spike in the signal it will have no influence)
• Sort the all median values and calculate the mean of the middle third. This will remove all values with very high/low back porch median (noise, tbc didn't work, sync lines) and only uses the "best third" for calculating the average
• This value is used instead of the default ire0 value when subtracting ire0. This is only done when the input is a complete field, not for a single value
• It deliberately does not do any calculation over multiple fields (floating average etc.), as formats like VHS-HQ or S-VHS have an offset every other field. This code is able to fully compensate that.

Proposal: As the VHS-HQ track_ire0_offset does not work reliable, as the track phase derived from chroma is not related to the track_ire0_offset phase and there doesn't seem to be away to figure the phase out, this code could be removed and selecting the VHS-HQ format could just result in turning on --ire0_adjust