Reduce latency by setting appsrc max-bytes to the block size

[ferjm]

With this patch I can notice a much lower latency running the theremin demo.

r? @Manishearth @sdroege

[sdroege]

This makes no sense for reducing latency :) It would mean that any additional data is dropped (see block property) instead of being queued up. Which would mean that you either should implement some kind of backpressure here or you're just producing too much data to begin with.

This needs some closer investigation why it has an effect at all. The appsrc is forwarding data as fast as it can (not once the limit is reached), and downstream is consuming data in real-time.

[ferjm]

We have a backpressure mechanism here. We stop feeding the appsrc with data if we reached max_bytes (which, after this patch, will happen every time we push a single rendered block into the appsrc). IIUC this would essentially be the same as setting the block property on. But IIRC we couldn't use the block property because we wanted to continue processing messages sent to the render thread while we are waiting for the sink to process the audio.

[sdroege]

Ah I see. You could as well set max-bytes to 1 if you only ever want to allow a single chunk btw :)

Makes sense then, I was forgetting that context.