Consider splitting OpenTK dependent classes

[tomrijnbeek]

🆙 Improvement proposal

Currently the library depends on OpenTK in a variety of places. However, many of its utilities stand by themselves and don't necessarily need to require OpenTK. Therefore it would be interesting to move all the OpenTK dependencies to a separate Bearded.Utilities.OpenTK package, so that choosing to include all of OpenTK in your project becomes optional (also, a Bearded.Utilities.Unity3D package could be considered mirroring the types for Unity).

Context

This should be considered in the context of #294 where we split off SpaceTime into a separate package, which then in turn would make sense to switch to System.Numerics types as underlying types (see #146).