Added lower-case Mipmap constants

[WestLangley]

Inspired by #6457 (comment).

It is _gl.LINEAR_MIPMAP_LINEAR, not _gl.LINEAR_MIP_MAP_LINEAR.

The boolean generateMipmaps uses the correct nomenclature.

This PR is a first-step. We can discuss the best way to proceed.

[Mugen87]

I think I would already use the new constants in the code base. Just introducing them in constants is a very small step.

[WestLangley]

This is ridiculous. The PR is too small?

Let's merge PRs and make progress.

[Mugen87]

PRs should represent meaningful bundles of work. I don't think it makes sense to just add a few constants to the lib. If you want to make progress, you should already introduce them in the rest of the code base.

BTW: #16671 has a similar problem. You have introduced .fromEquirectangularTexture() but the method is still not used once in the entire code base (and undocumented). It would have been better to directly replace all occurrences of EquirectangularToCubeGenerator and CubemapGenerator.

[WestLangley]

This PR adds support for the new nomenclature ("Mipmap"), while maintaining backwards-compatibility with the old ("MipMap").

I'll file another PR to use the new nomenclature in the source code, while leaving the examples as-is.

If that works, I'll update the examples to use the new nomenclature as a final step.

I think we want to maintain backwards-compatibility, so that means supporting both the new and the old nomenclatures.

Maybe someone has a suggestion for how the old nomenclature can be deprecated...

[Mugen87]

Again, I'm against this type of PR style. Remembers me at salami tactics....

[mrdoob]

TIL about salami tactics 😁

Doing perfect PRs can be a bit taxing, so I'm okay with solving issues in multiple PRs.

[mrdoob]

Maybe someone has a suggestion for how the old nomenclature can be deprecated...

Maybe we should turn them into getters and log a warning when accessed?

[WestLangley]

Maybe we should turn them into getters

Hmm... how is that done? 🤔