feat(performance) Multiple performance improvements and other fixes

[gmbeard]

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