Specular color texture is never passed to the PBR shader

At this line: https://github.com/mgsx-dev/gdx-gltf/blob/7d70839067ead66ec6107bc1be8c060c7c0fc7aa/gltf/src/net/mgsx/gltf/loaders/shared/material/PBRMaterialLoader.java#L167C7-L167C7

A specular color texture attribute is created but the texture is never passed to the shader.

@dar-dev i think to issue is here :

gdx-gltf/gltf/src/net/mgsx/gltf/scene3d/shaders/PBRShader.java

Line 315 in 7d70839

.get(PBRTextureAttribute.Specular))).textureDescription);

it should use PBRTextureAttribute.SpecularColorTexture instead of PBRTextureAttribute.Specular.

Could you please test it with your models to confirm it works and send a PR?

Tested using "PBRTextureAttribute.SpecularColorTexture":

com.badlogic.gdx.utils.GdxRuntimeException: Shader compilation failed:
Fragment shader:
0(1254) : error C1503: undefined variable "u_specularColorSampler"

	at net.mgsx.gltf.scene3d.shaders.PBRShaderProvider.checkShaderCompilation(PBRShaderProvider.java:633)

The issue here's the "specularColorTextureFlag" is never defined in the PBRShader.

Please, check our fix here: MikOfClassX@fdb7570

I can send a PR if it looks good to you..

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