Add frame timing data to ViewerStats to better characterize performance

[bennettgoble]

Current FPS reporting fails to adequately capture the qualitative experience of viewer performance. A session with high average FPS can still feel sluggish or stuttery due to irregular frame pacing or spikes in render time. To address this, we should add richer telemetry to the ViewerStats event that describes the distribution and variability of frame times.

Some suggestions on the data we should capture:

• tot_frame_time_std_dev - Standard deviation over past N frames (last viewer stat message?)
• tot_frame_time_p99 - 99th percentile over past N frames
• tot_frame_time_p95 - 95th percentile over past N frames
• frame_spike_count - Count of frames > 2x avg (?)

Including distribution information in percentiles will allow us to better get a feel for the qualitative experience:

• p99 shows rare spikes, which is good for spotting painful stutters
• p95 will demonstrate common hiccups and generally allow us to better judge general smoothness