Add GLVND OpenGL detection as a GLX(X11) alternative and remove the glut header

[vanfanel]

Hi there @TurningWheel

I wanted Barony running on X11-less systems, which offer the lowest possible latency.
Since Barony runs on OpenGL (not GLES, to my knowledge) and accesses the GL functions directly, the only way is to add suport for the modern and vendor-neutral GLVND GL, which is provided by MESA when built with GLVND support on: this way, we don't depend on any X11-related library, yet we can have full OpenGL without X11.

I have also removed the line trying to use glu.h in src/main.hpp, since GLUT is not used anymore (Good! Old GLUT stuff is not needed anymore) and trying to use glu.h causes building to fail on X11-less systems which of course don't have GLUT either.

So: This allows the Barony engine to be built on modern GNU/Linux systems without depending on X11.
For example, I have just built it on my Raspberry Pi4 running aacrh64 GNU/Linux with latest stable MESA (21.0.1 as of today) with no X11.
(Just remember that MESA with GLVND support is needed for building against GLVND OpenGL! Most modern common desktop distros support it anyway.)

[vanfanel]

Oh well, it won't run on the Pi4 because it only supports OpenGL 2.1, but runs great on my x86_64 laptop without X11, so I hope this gets merged anyway.

[perkinslr]

Oh well, it won't run on the Pi4 because it only supports OpenGL 2.1, but runs great on my x86_64 laptop without X11, so I hope this gets merged anyway.

FWIW, I have it running on a Pi4 (under X11). If you give the whole executable to clang for full LTO, enable fastmath and other similar optimizations, and run at fairly low resolution, it manages around 25fps. I think I could get it playable if I added a cooling fan to the Pi, and overclock it a bit.

I have also gotten it running under wayland without xwayland. There are some minor graphical issues (skybox is translucent, showing whatever window is behind), and it requires a few minor patches to get the mouse integration working. It does yell at me if I don't specify -DOpenGL_GL_PREFERENCE=GLVND to cmake.