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(https://github.com/rds1983/Myra/blob/master/src/Myra/Platform/IMyraRenderer.cs). Both interfaces have parents: ITexture2DManager and IFontStashRenderer

There's a sample demonstrating usage of Myra.PlatformAgnostic for the MonoGame: https://github.com/rds1983/Myra/tree/master/samples/Myra.Samples.PlatformAgnostic

Also there's more complicated sample for the Silk.NET: https://github.com/rds1983/Myra.Silk.NET