graphics/osgearth: Update to 3.5

Changes:
* New osgEarthCesium nodekit using the cesium-native SDK for 3D-Tiles
  integration
* Upgrades to the TerrainConstraintLayer for better meshing,
  performance, and support for 3D model constraints
* New TerrainMeshLayer for "bring your own" terrain surface meshing
* Integration of spdlog logging SDK (optional)
* Various updates to the osgearth_imgui panels
* Updated Triton and Silverlining shader sets

Reported by:	portscout