mono: make it find libgdiplus at runtime

Mono uses libgdiplus for it's System.Drawing implementation. libgdiplus
is already a buildInput but the build process doesn't store it in RPATH
of mono. So we have to do it ourself.
WIP: opentk: new package

OpenTK is a low-level C# library that wraps OpenGL, OpenCL and OpenAL.
WIP: Repetier-Host: new package (IT DOESN'T BUILD YET)

Repetier-Host is software for controlling RepRap style 3D-printer like
Mendel, Darwin or Prusa mendel. Works with most firmware types. It is
optimized to work with Repetier-Firmware Other working firmware is
Sprinter, Teacup, Marlin and all compatible firmwares.
opentk: working on it...

libgdiplus can be removed now that mono has it in RPATH (TODO: use
propagatedBuildInputs for libgdiplus in mono?).

OpenTK needs (at least) libGL.so and libGLU.so. So give it mesa.