.NET 6 Android Windowing support (#586)

* Initial pass of adding .NET 6 support for sustaining

* Get packaging working properly (afiak)

* lol

* SilkTouch: Stop making managed conv assumptions (.NET 5+), short names

* Triangle on .NET 6 on Android now works

* Fix old android lab app for fullscreen

* Fix bad ifdef

* CI fix?

* Review fixes