Consider upgrading to OpenTK 3.1

[ItEndsWithTens]

What I'm told was a miscommunication among the OpenTK developers was resolved a few months ago, and since then they've been accepting PRs with bug fixes and minor changes to the 3.0 branch of their project. A few days ago they prepared a release, and version 3.1 has been made available on NuGet.

The fix I think would be most useful regards creating GL contexts in Windows (a bug prevented RenderDoc from connecting properly), but there are several others. I'm not sure what impact any of them will have on Eto.OpenTK, especially since I know you've mentioned it's in use in at least one commercial project, and in the meantime I imagine the rest of us can use binding redirects, but I think it'd be worth considering an upgrade to the latest OpenTK release if the changes seem acceptable.