Performance:
- Adds dedicated thread for dequeueing and writing encoded packets
- Use native (BGRA) colourspace for NVFBC capture
  Using the native colourspace means NVFBC doesn't have to perform any
  conversion. In theory, this will improve capture performance
- Allow NvFBC to wait up to a maximum time for a new frame

Fixes:
- Adds eventfd for cancelling contexts without timers
  Contexts without timers require this to be cancellable with
  `request_stop()`.
- Fixes memory leak in h/w frame context initialization

Other:
- Adds metrics for video and audio encoding frame times
- Adds performance report generator script
- Allows NvFBC adjusted FPS to be enabled/disabled with ENV var
  `SHADOW_CAST_STRICT_FPS`. Defaults to `1`.
- Add FAQ