Add texture handling and refactor material functions

Added a new function `blinn_phong_specular_exponent_to_roughness` in `BSDF.h` to convert a specular exponent to roughness.
Added a `textures` member to the `shading_context_t` structure in `Material.h` for passing texture information during shading.
Added a new member `textures` in the `light_shading_context_t` structure in `Light.h` to hold texture information.
Added inline functions in `Texture.h` for handling texture entities, including `invalid_texture_entity`, `is_texture_entity_valid`, and `get_texture`.
Changed the texture-related members in `simple_lit_properties_t` in `SimpleLit.h` from pointers to `texture_entity_t` to better manage texture entities.
Changed the `sample_material_bsdf` and `sample_material_bsdf_pdf` functions in `Material.h` to accept a `shading_context_t` instead of individual parameters.
Changed the `mesh_load` function in `Mesh.c` to use the new roughness calculation and texture entity handling.
Changed the `path_trace` function in `PathTracing.c` to use the new structure and functions for handling materials and textures.
Refactored the `sample_material_bsdf` and `sample_material_bsdf_pdf` functions in `Material.c` to utilize the new `shading_context_t` structure.
Updated the `material_collection_init` function in `Material.h` to reflect changes in the material sampling functions.
Updated the `evaluate_bsdf_directional` and `evaluate_bsdf_const_sky` functions to use the new shading context structure.
Adjusted the `simple_lit_data_default` function in `SimpleLit.c` to work with the new texture handling approach.
Added texture entity handling in `Texture.c` for managing invalid texture entities.

header/Lighting/Light.h

@@ -24,6 +24,7 @@ typedef struct
 
     const bvh_tree_t* bvh_tree;
     const material_t* material;
+    const texture_collection_t* textures;
 } light_shading_context_t;
 
 #ifdef __STDC_ALIGN__