Eslint: Enforce eqeqeq in addons

[s-rigaud]

Description

I fix most eslint warnings for addons after improving th rules in previous PRs.

I tested all I can on my side especially Arcaball, nothing should be broken as I tried to double check the triple equality when testing against null as undefined == null but undefined !== null.

[Mugen87]

This PR breaks RGBMLoader (I suggest you do not alter the code from the embedded UPNG.js).

TBH, I'm a bit hesitating of making this change since it requires very detailed testing. Especially in modules that use the previous pattern all over the place.

[s-rigaud]

Yes I totally understand as no one can really plan all the side effects of those changes. I will just improve the JsDocs to reduce th number of eslint errors and warning in another PR then.