Improve ray marching in volume rendering examples

[mrdoob]

Description

This PR improves the ray marching algorithm in webgl_volume_perlin, webgl_volume_instancing, and webgl_volume_cloud.

The previous implementation used a DDA-like approach, which could be unstable and produce artifacts.
This PR replaces it with a more stable and correct ray marching loop that steps along the camera direction.

[gkjohnson]

Do you have images of the specific artifacts you're trying to address? Using a fixed length step feels like a less optimal approach, at least for non-interpolated voxelized data like in the volume instancing demo. It also looks like the previous approach wasn't really using DDA unless I'm misunderstanding?

[mrdoob]

In the perlin example you can see less artifacts:

https://threejs.org/examples/webgl_volume_perlin.html
https://raw.githack.com/mrdoob/three.js/dev/examples/webgl_volume_perlin.html

In the instancing one you can see less gaps in the volume:

https://threejs.org/examples/webgl_volume_instancing.html
https://raw.githack.com/mrdoob/three.js/dev/examples/webgl_volume_instancing.html