Reload WGL entry points in WinGLContext()

This fixes AccessViolationException when creating multiple contexts on
Intel hardware.
Serialize Sdl2NativeWindow methods

This change, along with moving DestroyWindow() to the Dispose() method
fixes crashes on shutdown.
Merge branch 'sdl2' of https://github.com/thefiddler/opentk

Conflicts:
	Source/OpenTK/Platform/SDL2/Sdl2NativeWindow.cs