Move glad into another repository

[RiscadoA]

Problem

Currently glad is a submodule of our repository.
glad is a library whose code must be first generated according to some configurations we set. Only then is it really built and linked into the engine.

The actual problem lies in the glad code generation being implemented in Python, and thus Python becomes a necessary dependency to build Cubos (ew).
The glad team recommends users to simply use their web interface to generate the code, and then include the generated code in our source tree.
That is what we did previously, but we find it ugly to just have external files laying around on our repo like that.

Solution

We should create another repository, cubos-glad or something, where we keep the external files generated with glad's web interface, and the relevant instructions in a README next to it.
We can then add that repo as a submodule to the main engine repo, and thus avoid needing Python to compile everything, while keeping the repo clean.

Extras

We could also do this for header only libraries, such as stb_image. We currently have that file in our engine/lib directory, counting towards our line count and contributor stats, which is a bit odd.
We could also move it to a cubos-stb repo, and then add it as a submodule here.