Consumer driven streams

Sometimes it's better if the consumer is the entity that drives the generation of new buffers in a stream.

An example of this is a screen cast where the consumer is a headless compositor that doesn't have any monitor to v-sync to. This is generally currently handled by applying an arbitrary refresh rate.

By letting the consumer being in control of when new buffer/frames are to be produced, one could effectively v-sync the producer (e.g. a headless compositor) to the monitor of the consumer (e.g. across a network barrier or in a local viewer setup).