Add support for GLTF extension KHR_materials_emissive_strength

[RevoluPowered]

This reads the multiplier for the emission strength from GLTF files.

This is fairly universal and is required for blender's emission multiplier to be imported.

The value is a simple universal multiplier and is unitless.

Sponsored by The Mirror.

Makes emission just work by default from GTLF, which is great!

[RevoluPowered]

Specification of this extension:
https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_emissive_strength/README.md

[aaronfranke]

Link to the spec: https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_emissive_strength

The code for this is very simple and in line with some of the other KHR material extensions also handled here.

(btw, pbr_spec_gloss_extensions is incorrectly named, it's actually all extensions on the material).

[RevoluPowered]

Link to the spec: https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_emissive_strength

The code for this is very simple and in line with some of the other KHR material extensions also handled here.

(btw, pbr_spec_gloss_extensions is incorrectly named, it's actually all extensions on the material).

Yeah I saw this it definitely would be good to refactor at some point, wanted to keep this super minimal for now.

[clayjohn]

Looks good to me. For future reference, without this extension GLTF only supports setting emission texture and emission color (clamps to 0.0-1.0). This extension maps perfectly on to how we handle emission in the StandardMaterial3D

[RevoluPowered]

Looks good to me. For future reference, without this extension GLTF only supports setting emission texture and emission color (clamps to 0.0-1.0). This extension maps perfectly on to how we handle emission in the StandardMaterial3D

I was really surprised about this being super standard and just a very simple multiplier. A lucky find :)

[YuriSizov]

Thanks!