videoconverters, videotestsrc, etc: Wrongly claim to do multiview-mode conversion

Submitted by Sebastian Dröge @slomo

Link to original bug (#776172)

Description

gst-launch-1.0 videotestsrc ! videoconvert ! "video/x-raw,multiview-mode=top-bottom" ! fakesink -v

Problem here is that basetransform and everything else is doing a caps query downstream, and then intersect. The field not existing in the upstream caps then causes the field to get the downstream value.
Which would be no problem if the absence would not mean anything. It however means the same as multiview-mode=mono.

So all these elements will have to ensure that they will handle it like mono and never ever output different multiview-mode unless the input had that different multiview-mode already.