Add renderer parameter to GLTFExporter

[fernandojsg]

Fixes #11962

[mrdoob]

Hmm... This renderer dependency doesn't feel right.

Considering that gl constants do not change, I think it would be better to create a map like in GLTFLoader.

https://github.com/mrdoob/three.js/blob/dev/examples/js/loaders/GLTFLoader.js#L771-L791

Same thing for WebGLUtils...

[fernandojsg]

I had it like that in the beginning but I changed it to reuse the WebGLUtils class. For me it also looks strange that we need to keep the list of constants everywhere we use it instead of having a WebGLUtils or so we we could do the match.
Maybe having the list of constants with it number directly instead of accessing the webgl context? Similar to https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Core/WebGLConstants.js

Also please note that if the following iterations of the exporter we'll need the renderer to access the textures directly, transform them and read the pixels back to store within the file.

[mrdoob]

Maybe having the list of constants with it number directly instead of accessing the webgl context?

Sounds good to me 👌

Also please note that if the following iterations of the exporter we'll need the renderer to access the textures directly, transform them and read the pixels back to store within the file.

Lets try to keep the exporters renderer agnostic until is just not possible.

[fernandojsg]

Ok, I'll close it and create another PR with the proposed changes