[WIP] Emulation of 3D textures

[izaid]

The current specification of WebGL doesn't support 3D textures. This PR adds emulated 3D textures, by sending them to the GPU as a 2D texture and providing an appropriate look-up function.

This is based off of work that I and others did in VisPy, see https://github.com/vispy/vispy/blob/73ff8851c005a1c678dbf5879a7dbf3fc2747f9a/vispy/gloo/texture.py

[codecov-io]

Current coverage is 86.17% (diff: 0.00%)

Merging #43 into master will decrease coverage by 0.92%

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