Major refactor of the demos/ directory, accomplishing a number of goals:

1. Allowing easy switching between OpenGL and Vulkan variants.
2. Supporting both GLFW and SDL2, chosen at compile time. The window
   management itself was made entirely modular for this purpose.
3. Refactor all of the demo programs atop these new abstractions.
4. Building them correctly and automatically (via meson).

I also plan on factoring out the nuklear code to its own helper, but I
avoided making that part of the same commit to prevent it from blowing
up any further.