rdp: Allow disabling RemoteFX codec

There are currently compatibility issues between FreeRDP's implementation
of the RemoteFX codec and Microsoft's implementation.

Perhaps this will be fixed in the future and this option can go away,
but for now it's necessary to have a way to disable the codec if the
windows client is going to be connecting to a weston server.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>