video: Bottom-field-first is a bad default

Submitted by Sebastian Dröge @slomo

Link to original bug (#775231)

Description

For interlaced video, we have a flag to specifically signal that we have top-field-first. However everything except for NTSC DV is top-field-first, so that would make a much better default (and almost no code is setting the flags anyway).

I'm not sure how to best change this in 1.x though, any ideas?