Add a minimal example in C

[linkmauve]

This is a direct port of examples/canvas_minimal, using GLFW instead of SDL2 and written in C to show how to use this binding.

[pcwalton]

Thanks! How about putting this in examples/, to go alongside the existing macos_app C example?

[linkmauve]

I moved it to examples/glfw_app/ to match the macOS example.

I’m planning on moving pathfinder_c to use lu_zero’s cargo-c instead of bindgen directly, this will give the benefit of creating a .pc file that can then be installed, and let other software use Pathfinder’s C API as they would with any other C library, with pkg-config.