- Gave glTFs created from quantized-mesh terrain tiles a more sensible material with a `metallicFactor` of 0.0 and a `roughnessFactor` of 1.0. Previously the default glTF material was used, which has a `metallicFactor` of 1.0, leading to an undesirable appearance.
- Reported zero-length images as non-errors as `BingMapsRasterOverlay` purposely requests that the Bing servers return a zero-length image for non-existent tiles.
- Added an `Axis` enum and `AxisTransforms` class for coordinate system transforms
- Added support for the legacy `gltfUpVector` string property in the `asset` part of tilesets. The up vector is read and passed as an `Axis` in the `extras["gltfUpVector"]` property, so that receivers may rotate the glTF model's up-vector to match the Z-up convention of 3D Tiles.