Not tested, could have errors

Could correct possible issues on OpenGL 1.1

- Review RPI compilation (core_drop_files not supported)
- Review ImageFormat(), some issues
- GetTextureData() reviewed for RPI

...and reviewed some details

Some people might find this handly

...to avoid issue on Arch Linux

We need to grab the OS name before determining what the path should be.

SetMaterialTexture() and UnsetMaterialTexture()... too shader
dependant...

Moved PBR material loading to example, right decision?