[OpenTK] Fixed compilation issue on desktop
[SDL] Call SDL.GL.SetAttributes before window construction

SDL2 requires OpenGL attributes to be set before the window is
constructed. Without this, context construction on Android will fail.
[SDL] Use SDL.GL.ResetAttributes() on SDL 2.0.2+
[SDL] Call SDL.GL.SetAttribute() before window construction
[SDL] Make Sdl2GraphicsContext.ClearGLAttributes() private
[Examples] Fixed OpenGL ES 1.1 color pointer format