h265parse: Detect height change on field-based interlaced files

The first time update_src_caps is called, there's no frame parsed yet, therefore we don't know whether the file has alternate-field interlacing mode. If we run it again after we have a frame, it might be that now we have the SEI pic_struct parsed, and therefore we know that it's field-based interlaced, and therefore the height must be multiplied by two. Earlier on this was not detected as a change.