b51d32a812
The framerate for screen cast sources was set to variable within 1 FPS
and the framerate of the monitor being screen casted. This meant that if
the sink didn't match the framerate (e.g. had a lower max framerate),
the formats would not match and a stream would not be established.
Allow letting the sink clamp the framerate range by setting it as
'unset', allowing it to be negotiated.
(cherry picked from commit 041be7c565)