videoflip: fix possible crash when changing video-direction/method while running

commit c46e132d

videoflip: fix possible crash when setting the video-direction while running

A classic case of not enough locking.

One interesting thing with this is the interaction between the
rotation value and caps negotiation.  i.e. the width/height of the caps
can be swapped depending on the video-direction property.  We can't lock
the entirety of the caps negotiation for obvious reasons so we need to
do something else.  This takes the approach of trying to use a single
rotation value throughout the entirety of the negotiation and then
subsequent output frame in a kind of latching sequence.

commit 2e9b7e65

tests: add tests for videoflip

Fixes: #792 (closed)