Better fix for recent Draco changes

[bghgary]

See forum: https://forum.babylonjs.com/t/setting-the-value-0-to-the-defaultnumworkers-from-dracocompression-throw-an-engine-error-since-6-21-1-version/44323

• Undo changes to vertex buffer so that engine is no longer nullable.
• Factor out GetFloatData function so that it can be used in DracoCompression.
• Update DracoCompression code to use VertexBuffer only when it's necessary.

[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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