avviddec, video.c, h265parse: Workaround for broken field-based interlaced encoders

Some encoders (e.g. Makito) have H265 field-based interlacing, but then also specify an 1:2 pixel aspect ratio. That makes it kind-of work with decoders that don't properly support field-based decoding, but makes us end up with the wrong aspect ratio if we implement everything properly. As a workaround, detect 1:2 pixel aspect ratio for field-based interlacing, and check if making that 1:1 would make the new display aspect ratio common. In that case, we override it with 1:1.