(WIP) glTF export: improve material export

[querielo]

This pull request improves the material export in the glTF exporter.

• added support for exporting normal maps, metalness maps, and emissive maps.
• added support for exporting textures with texture transforms (tiling, offset, and rotation).
• added support for exporting alpha test and alpha cutoff values for materials that use the "none" blend mode.

Please review. Thank you!

[willeastcott]

This is a great contribution! It's great to see the glTF exporter get some love. Hey, @marklundin, got any idea how we can better make use of the glTF exporter in the Editor? Maybe a context menu option when right clicking on entities in the Hierarchy view?

[mvaligursky]

related to #5001

[mvaligursky]

The code assumes the textures have their channels matching the glTF specification here. We should test for that (materials in PlayCanvas specify which channel is used) and only export the textures if the channels match.

Ideal solution in the future would be that if the channels do not match, we build the required textures from source textures.

Also, the normal map can have this format, which we should probably warn about and not export either, till we have code to convert it:

engine/src/platform/graphics/constants.js

Lines 1049 to 1056 in 47d1f4c

	/**
	* Texture stores normalmap data swizzled in GGGR format. This is used for tangent space normal
	* maps. The R component is stored in alpha and G is stored in RGB. This packing can result in
	* higher quality when the texture data is compressed.
	*
	* @type {string}
	*/
	export const TEXTURETYPE_SWIZZLEGGGR = 'swizzleGGGR';

The goal here being that we can only export valid glTF and print a warning otherwise.

[marklundin]

Yep great point @willeastcott, would love to surface this in a more intuitive way for users. It's a great piece of functionality and adds a lot of value. I'll have a think about this.

Great PR too @querielo 🔥

[querielo]

Ideal solution in the future would be that if the channels do not match, we build the required textures from source textures.

I'll fix it later when I have free time (our content is usually loaded from .glb-files).

[mvaligursky]

Please try to avoid make this PR larger and keep it focused on the material export improvements this PR is about, as this is getting hard to review now. Follow up work is awesome, but a separate PR please.

[mvaligursky]

Approving, tested and works well, well done and thanks!