Add KHR_draco_mesh_compression support to glTF Exporter

[alexchuber]

This PR adds the ability to export to glTF using Draco encoding. Some notes:

KHR_draco_mesh_compression

• Usage:

BABYLON.GLTF2Export.GLTFAsync(scene, "fileName", {  meshCompressionMethod: "Draco" });

• Using Draco compression will increase export time, especially if the scene to export contains many unique meshes.
• A primitive's vertex data + indices are either fully Draco-compressed or not. As such:
  • No uncompressed "fallback" data is exported if a primitive is Draco-compressed. This can be improved in future iterations.
  • KHR_draco_mesh_compression will be a required extension if any primitive in the glTF is successfully Draco-compressed
• The encoder options used in the glTF exporter are defined as follows:

const DefaultEncoderOptions: IDracoEncoderOptions = {
    decodeSpeed: 5,
    encodeSpeed: 5,
    method: "MESH_EDGEBREAKER_ENCODING", // Or, if primitive contains morph targets, "MESH_SEQUENTIAL_ENCODING"
    quantizationBits: {
        POSITION: 14,
        NORMAL: 10,
        COLOR: 8,
        TEX_COORD: 12,
        GENERIC: 12,
    },
};

glTF Exporter

• Creation of bufferViews and accessors are all done in the same way, through an intermediate class to help track relationships
• Final binary buffer is no longer built on-the-go, but instead only once requested via generateBinaryAsync
• Fixed glTF validation errors related to byte alignment, among other small buffer-related warnings

[bjsplat]

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[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bghgary]

Some minor comments. Looks good otherwise.