interlace does possibly-wrong passthrough when field-order is missing from upstream caps
The following discussion from !1349 (merged) should be addressed:
field-order is optional in the caps. This means that, if we're in a non-telecine mode and we have TFF upstream and top-field-first=FALSE in interlace (or the other way around), AND field-order isn't mentioned in the caps, we will do passthrough here and end up outptuting wrong data. Must detect missing field-order info and not do passthrough in that case, but instead check the GstVideoBufferFlags.