Add the ability to use OpenRV as an NDI client

[markreidvfx]

Now that at least commercial RV can use NDI as a presentation device it would be amazing if we could use OpenRV as an NDI a client too.

Our use case would be for non-interactive viewing of a live review sessions. We have many linux users and the options for viewing NDI streams are limited. Being able to view a NDI stream in a familiar review tool has a lot of benefits. Having access to OCIO so the NDI streams can be color managed would be extremely useful.

Its not entirely clear how this would work with RV's frame based playback architecture or how audio would work. It could just be a live feed that simply ignores the current playback settings.