Local media fixes

* More frame rate limiting using drop-only=true and max-rate=fps
* Removed unnecessary conversion elements from OwrVideoRenderer as it uses glimagesink which can do colour space conversion and scaling itself, and it performs very well so framerate isn't really a problem
* Make OwrVideoRenderer queue downstream-leaky, just for good measure. If it was blocked up for 3 buffers, something odd is going on with the load/scheduling and dropping is probably a good choice.

I tested the OwrVideoRenderer and OwrLocalMediaSource stuff with a self-view application that applied frame rate and resolution restrictions. I tested OwrImageRenderer with the daemon and all seem to work OK.
Absolutely. I was looking at glimagesink to see if there was some way to disable video as videobalance is software. I thought as a quick hack we could make a shader in client-draw that makes all pixels black.