Load tiles with fade-in

[OmarShehata]

This came up on the forum. I initially thought it might just be something Cesium can do, but @ggetz suggested it might better to be part of the spec, to define a way to control how the tiles load, whether they should just pop in as they currently do or fade in.

I think either way it would be a simpler thing to add that might make everything look a lot nicer.

[pjcozzi]

I don't think this has anything to do with the spec; this should be a runtime engine decision. Currently tracking in CesiumGS/cesium#3241