Default to colorBlendMode=HIGHLIGHT for Cesium3DTiles

[na9da]

Reported here #7116

Currently Terria defaults to colorBlendMode=MIX for 3D tilesets whereas Cesium's own default is HIGHLIGHT.

MIX is a poor choice for default because when setting a style value it will multiply the style value with tileset color by colorBlendAmount resulting in a washed out look. We should instead default to HIGHLIGHT (same as Cesium).

Need to check how this interplays with our feature selection for buildings datasets.