fill_analog_frame_ends: mod2 field height required on pre-cropped progressive material.

[JustinTArthur]

In case fill_analog_frame_ends encounters interlaced frames, it always attempts field separation. This breaks when the function is fed a progressive clip that's been cropped to something other than mod2 half-height. Ideally, fill_analog_frame_ends is applied before crops, but we can't guarantee users won't try.