Fix problems with QuantizedMeshLoader-generated glTFs

[kring]

Before this PR, the glTFs generated from quantized-mesh tiles were technically invalid. Our game engine clients weren't too fussed about these problems, which is why we didn't notice.

Fixed the following problems:

• The asset.version field was not set.
• The byteLength of the buffers was not set. Most clients just used cesium.data.size() anyway.
• The target type of the bufferView used to store indices was incorrectly set to ARRAY_BUFFER instead of ELEMENT_ARRAY_BUFFER.
• The bufferView used to store indices had its byteStride property set, which is not allowed.
• The glTF did not have any scenes.