synchronous frame forwarding

[dexterlb]

Currently, the FrameForwarder works on a "hand over frames asap" basis, dropping any frames that aren't requested in time. This is great for streaming because it has extremely low latency. However, the user might want to use some sinks for recording - in these cases, they may want to be able to request a "synchronous forwarding" mode that buffers up to NumAllocatedFrames frames and sends them over in a queued manner instead.

Here's one idea of how this would work:

• GetFrameForReading should:
  • pop the frame queue (blocking)
• FinishedReading should:
  • push to the recycle bin
• GetFrameForWriting should:
  • try getting a frame from the recycle bin
  • if no frame was available return nil and allow the caller to framedrop
    • log a framedrop
  • unmark the frame for recycling and return it
• FinishedWriting should:
  • push to the frame queue