WebGLRenderer: Stop rendering Geometry

[Mugen87]

With this PR, WebGLRenderer only supports BufferGeometry. If users still use Geometry in their scene, they have to manually convert their geometries to BufferGeometry before rendering like so:

const geometry = new THREE.BufferGeometry().fromGeometry( legacyGeometry );

I've tested all WebGL examples, the docs and the editor with this change.

Related #15387

[aardgoose]

Can/will all the ray casting support for Geometry() in Mesh() and Line() be removed as well?
Also computeLineDistances() in Line() and LineSegments()?

[Mugen87]

I think so. But please discuss such topics here: #15387. This PR is focused on WebGLRenderer.

[Mugen87]

Should we give it a go in this dev (r102) cycle?

[Mugen87]

Um, #16928 and similar reports on the forum/ stackoverflow showed that still a lot of people create Points with Geometry.

Before merging this PR, I suggest to announce the stop of Geometry support in WebGLRenderer with a console warning. It can be logged when projectObject() encounters Geometry while processing the scene. One or two release later, this or a similar PR can be merged.

[Mugen87]

Closing due to merge conflicts. It might be easier to setup a new PR when everybody feels okay with stopping support of Geometry in WebGLRenderer. As mentioned before, the first change in this process should be an announcement of this change with a console warning.