Support for .Net 5

[meschi89]

Please add support for .Net 5.0

• https://devblogs.microsoft.com/dotnet/introducing-net-5/
• https://devblogs.microsoft.com/dotnet/the-future-of-net-standard/

I got the following message from Nuget:
myproject.csproj: [NU1701] Package 'OpenGL.Net 0.8.4' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework 'net5.0'. This package may not be fully compatible with your project.

[daemitus]

Clone the project as a submodule, and reference OpenGL.Net_netstd2.0.csproj