And require them to follow the `$varname` (can't be $(varname) as
parenthesis have another meaning in those expressions).
Still accept "duration" and "position" as varname for backward compat
but update our scenarios anyway.
This way it is clear that you are using a variable reading the scenario
and we can verify that what the scenario writer intents is to use an
already set variable.
In case of fast-forward scenario, the playback-time is not set properly
as per increase in the rate. This is resulting in short media files of duration
less that 15 seconds to fail.
https://bugzilla.gnome.org/show_bug.cgi?id=754151
