They were disabled because they failed to build,
but this patch set fixes the build on Linux and macOS.

This doesn't apply to the AppVeyor build on Windows yet;
it currently fails at linking with OpenAL.

libEGL and libGLESv2 have been renamed on latest Raspbian Stretch
version (also included on latest Jessie upgrade).

It seems to be done to avoid conflicts between official Mesa libs and
Broadcomm provided ones.

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?