videorate: Detect framerate if not forced to variable downstream

videorate: Detect framerate if not forced to variable downstream

In case upstream does not provide videorate with framerate information,
it will detect the current framerate from the buffer it received,
but if downstream forces the use of variable framerate (most probably
through the use of a caps filter with framerate = 0 / 1), videorate will
respect that.

And add some unit tests

https://bugzilla.gnome.org/show_bug.cgi?id=734424