3D Tiles - Request scheduling improvements

• Time slice tiles that are processing
• Explicitly cancelling requests that go out of view
• Cull received tiles that are now out of view
• Tweaks for better http2 performance. Some notes in https://github.com/AnalyticalGraphicsInc/cesium/pull/5317
• Stagger renders to allow more request throughput: https://groups.google.com/forum/#!topic/cesium-dev/WQ4mgLcoBqI
• Debug visualizations to see load order.
  • Colorize by load order
  • Secondary camera see loaded tiles from a better vantage points
• Statistics / tracking improvements
  • Statistics for average load time
  • Statistics for number of cancelled requests
• Evaluate performance for smaller or larger tile sizes
• Prioritize based on center of screen: https://github.com/AnalyticalGraphicsInc/cesium/pull/6243#issuecomment-401496089
• How to mix and match different prioritization modes. Additive tiles use distance, replacement tiles use SSE, terrain uses its own priority. Is there any way to normalize all values to a 0-1 range?
• Fix 3D Tiles starving terrain/imagery: https://github.com/AnalyticalGraphicsInc/cesium/issues/5954
• Implement Request pool to avoid allocating many Request objects.

[loshjawrence]

non-skipLOD alpha blending, manual zoom

[loshjawrence]

Before and after of load time colorization heat map (again, it’s reversed: black/blue=first load, red/white=last load) colorized by time since new scene load.