GLTFLoader: Allow textures which share the image source

[takahirox]

Description

This PR allows GLTFLoader to create textures which share the same image source if multiple textures point to the same texture image source in glTF def.

Benefits:

• Memory optimization: If multiple textures point to the same texture image source in glTF, texture image source will be shared and duplicated texture image source upload to GPU texture will be avoided.

Changes:

• Breaking change: Change the second argument of parser.loadTextureImage() from texture source def object to source index. It can break external glTF loader plugins. Source index is needed for caching described below.
• Add loadImageSource(sourceIndex, loader) to parser which caches textures with source index. If cache misses, it creates a new texture from a source def. If cache hits, it clones the cached texture (so that source is shared).
• Some minor clean up

Side note:

• If Texture allows instance creation from source, the code can be more straightforward? Like

const source = await parser.loadImageSource(sourceIndex, loader);
const texture = new Texture(source);
texture set up here...

/cc @donmccurdy

[takahirox]

Note: Two e2e tests fail

List of failed screenshots: webgl_materials_envmaps_hdr_nodes webgl_materials_envmaps_pmrem_nodes

But they don't use GLTFLoader so I think unrelated.

[mrdoob]

Thanks!

[takahirox]

Added a note about the breaking change to the migration guide

https://github.com/mrdoob/three.js/wiki/Migration-Guide

Please feel free to rephrase what I wrote because English isn't my first language.

[mrdoob]

@takahirox Thanks! Tweaked: https://github.com/mrdoob/three.js/wiki/Migration-Guide/_compare/80ed55965a9bb36b9f8b3c9034da148c0399d607...fb50a67c9ef61b4bd9933f68f37c5f8475836676