anchor.diff:  [<AllowNullLiteral>]
   type ContextHandle(handle : IGraphicsContext, window : IWindowInfo) =
       static let current = new ThreadLocal<Option<ContextHandle>>(fun () -> None)
  +    static let contextError = new Event<ContextErrorEventHandler, ContextErrorEventArgs>()
   
       let l = obj()
       let mutable debugCallbackInstalled = false
  
positive.code:                 | _ -> None
  
          and set v = current.Value <- v
  
      member x.GetProcAddress(name : string) =
          (handle |> unbox<IGraphicsContextInternal>).GetAddress(name)
  
negative.code: open OpenTK.Graphics.OpenGL4
  open Aardvark.Base
  
  type ContextErrorEventArgs(msg : string) =
      inherit EventArgs()
  
      let mutable retry = false
      member x.Retry 
          with get() = retry
          and set (value : bool) = retry <- value
  
      member x.Message = msg
  
  type ContextErrorEventHandler =
      delegate of obj * ContextErrorEventArgs -> unit
  
  /// <summary>
  /// a handle represents a GL context which can be made current and released
  