Using platform agnostic version of Myra

Myra.PlatformAgnostic is version of Myra that isnt tied to any game engine.

It is available at the nuget: https://www.nuget.org/packages/Myra.PlatformAgnostic/

In order to use it, it is required to implement interfaces IMyraPlatform and IMyraRenderer.

For the latter, among methods DrawSprite and DrawQuad, only one should be implemented. The other could throw NotImplementedException. The property RendererType should return which method is implemented.

Samples that work through DrawSprite: https://github.com/rds1983/Myra/tree/master/samples/Myra.Samples.PlatformAgnostic https://github.com/rds1983/Myra/tree/master/samples/Myra.Samples.Silk.NET.TrippyGL

Sample that work through DrawQuad: https://github.com/rds1983/Myra/tree/master/samples/Myra.Samples.Silk.NET