This improves the get_line0_fallback function used by --fallback_vsync to use some more tricks to find the line0 pulse: It will not only consider pulse length, but also pulse distance to detect different parts of the blanking area. The old method (just return last vsync pulse before a long pulse) is the last fallback when the more advanced methods do not work.
Additionally get_line0_fallback will also detect field type and determine how confident the field type detection is and return these values. Those are passed to get_first_hsync_loc that will use them if either confidence of get_line0_fallback is very high or if there was no previous field detected