avviddec: Constantly "renegotiating" with alternate frame stream
*Reproduceable with validate.file.playback.fast_forward.cm5000_hevc_1080i_colorbars_ts
The problem is that update_video_context (ffmpegdec, context, picture)
returns TRUE (i.e. the format changed) even though it didn't, because it has no idea about alternate
(single-field) interlacing style.
As a result it constantly renegotiates itself and sets latency.
cc @vivia