point.pixelSize limit due to WebGL limit on some hardware

I explained an issue that we encountered when using point.pixelSize to portray time-series data by growing the pixelSize over time. It reached a limit at 60 pixels on this machine.

See https://github.com/TerriaJS/terriajs/issues/991

@kring found that it is due to "Rasterizer, Aliased Point Size Range" being too small on some systems. He explained there a potential workaround to implement in Cesium (switch to billboard when point size reaches its limit).

Start by reading the linked TerriaJS issue #991 and the discussion of the WebGL Rasterizer, Aliased Point Size Range. Trace how Cesium handles point.pixelSize as it approaches the hardware limit. Done means point rendering continues past that limit using the proposed billboard workaround.

Written by the indexing model from the issue text.