Gwen itself converted to netstandard 2.0

Gwen.Renderer.OpenTK is updated to OpenTK 4.1.0 and converted to netcoreapp3.1

Fixed few bugs, related to text lables.

Adapted Gwen OpenTK Renderer to not use legacy OpenGL: Uses VBOs and shaders instead of client side vertex arrays and fixed function shading pipeline. Draws triangles instead of quads.

Fork of HappyEmu's https://github.com/HappyEmu/gwen-nolegacy-opentk-renderer

that is a fork of https://code.google.com/p/gwen-dotnet/

Original code is C++ written by Garry Newman: https://github.com/garrynewman/GWEN