arbitrary fade for OSG2

[nzjrs]

From Liang. The current implementation

https://github.com/loopbio/FreemooVRPrivate/blob/ffb62305ef8331c98d819a6f6059a08d06cb826a/src/StimulusOSG2.cpp#L418

could be extended relatively easily with a global fade_final value, and if fade_time <= 0 then just snap to that