Squashed 'Source/OpenTK/Platform/SDL2/sdl2-cs/' content from commit b434bac

git-subtree-dir: Source/OpenTK/Platform/SDL2/sdl2-cs
git-subtree-split: b434bacba859a036b11ec3ac76c3baa058764769
Merge commit 'ed45dddb98c8250a33505c953b6c60bf7a854ffb' as 'Source/OpenTK/Platform/SDL2/sdl2-cs'