Fix czm_specularEnvironmentMaps datatype

[XiaoHu1994]

Description

The first parameter of czm_textureCube needs to be of type samplerCube

#ifdef SPECULAR_IBL
vec3 sampleSpecularEnvironment(vec3 cubeDir, float roughness)
{
#ifdef CUSTOM_SPECULAR_IBL
float lod = roughness * model_specularEnvironmentMapsMaximumLOD;
return czm_textureCube(model_specularEnvironmentMaps, cubeDir, lod).rgb;
#else
float lod = roughness * czm_specularEnvironmentMapsMaximumLOD;
return czm_textureCube(czm_specularEnvironmentMaps, cubeDir, lod).rgb;
#endif
}

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• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @XiaoHu1994!

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