- 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?

Namely:
- GenImageHorizontalV
- GenImageHorizontalH
- GenImageChecked
- GenImageWhiteNoise
- GenImageCellular

The gradient implementation may be a bit naive, for example it doesn't do any gamma correction.