GLTF Serializer rework.

[SergioRZMasson]

This PR changes fundamentally how the Babylon serializer to GLTF works. It changes the way the serializer handles left hand to right hand conversion.

Left hand to Right hand conversion

We no longer set mesh nodes with a negative scaling to handle left vs right hand conversion. We instead change every single vertex information when needed (normal, positions, and tangents) if the mesh was created in a left handed scene. We do the same for animation data and node transforms.

This should help decrease the file size of the exported glTF by removing the need for a root node to convert handedness, essentially reverting #13909.

Cameras and Lights

We now collapse cameras and lights with their transform nodes that are added by the Babylon importer. The goal is for the scene to be able to roundtrip without infinitely expanding.

Preserve vertex data

We now avoid as much as possible changing the type of vertex data. If the Babylon scene is holding vertex data using a particular type we will avoid at all cost converting it to float or to any other type. We might apply transformations to the data (such as left to right hand system conversion) but those modifications will be done in place, avoiding as much as possible creating a new buffer.

We also allow meshes that share the same geometry to point to the same accessors in the generated GLTF, se we no longer do any buffer duplication.

This should help significantly decrease some glTF file sizes on roundtrip, as we no longer make copies of buffers nor needlessly convert smaller buffer types to float buffers.

Forum issues

This PR also fixes a couple of bugs from the forum:

Wrong flipping of animated cameras
Inconsistent transform serialization
Instances exporting as separate meshes and #14056

[bjsplat]

Visualization tests for WebGL 1 have failed. If some tests failed because the snapshots do not match, the report can be found at

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/15869/merge/testResults/webgl1/index.html

If tests were successful afterwards, this report might not be available anymore.

[SergioRZMasson]

I'm temporarily converting this back to draft since we are waiting on @ryantrem, the plan is to merge it next week.