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