access textures inside a gltf material content handler

Description

When loading assets we can pass a container handler to hook into loading process. In case of gltf material, we can pass preprocess, process, postprocess hooks.

As for now, the only way to access textures is inside the process hook
https://github.com/playcanvas/engine/blob/de6e47f760faa54ad0b8f1655c57e1e302bff617/src/framework/parsers/glb-parser.js#L1724
However, passing a process hook then skips the default implementation of createMaterial
https://github.com/playcanvas/engine/blob/de6e47f760faa54ad0b8f1655c57e1e302bff617/src/framework/parsers/glb-parser.js#L1717
Unfortunately createMaterial is currently not exposed to the public API, so we can not apply custom processing steps on top of the default implementation. This prevents custom gltf extension that rely on custom textures from adding their functionality.

Suggestions

1. Pass textures also down to the postprocess phase
2. Expose createMaterial to the public API somehow https://github.com/playcanvas/engine/blob/de6e47f760faa54ad0b8f1655c57e1e302bff617/src/framework/parsers/glb-parser.js#L1168
3. Preferably also expose extractTextureTransform https://github.com/playcanvas/engine/blob/de6e47f760faa54ad0b8f1655c57e1e302bff617/src/framework/parsers/glb-parser.js#L899

Links

• https://forum.playcanvas.com/t/how-to-create-a-custom-gltf-plugin/32793/4
• https://github.com/giniedp/nw-models/blob/85121cf37735bacbc762401c98c438d759b14729/packages/viewer/src/playcanvas-viewer/viewer.ts#L116