Additional builtin variables

[ori-sky]

Would it be possible for some additional builtin variables to be added to this plug-in as described below? My motivation is that they would allow shaders to be automated in a sense based on stream-related actions.

• Elapsed time since started outputting (whether streaming, recording, or otherwise). This would allow shaders to be written as a transition for the beginning of a stream or recording.
• Elapsed time since scene became active. This would allow some transitions to be performed that may not be possible with OBS’ current transition system.