Geometry support

[henry-brisson]

Relevant info:

• https://discourse.threejs.org/t/three-geometry-will-be-removed-from-core-with-r125/22401
• In BufferGeometry, a vertex is saved using the position attribute
• Faces are generated automatically from vertices but that also means repeating vertices since each face requires three of them
• To reuse vertex indices for faces, the setIndex method of BufferGeometry needs to be used
• If all else fails, use legacy Geometry to convert, or maybe learn from it
• Concern: morph targets
• For meshes with multiple materials see: https://threejs.org/docs/index.html#api/en/core/BufferGeometry.groups - to slice geometry into groups that belong to different material indexes