[GLControl] Use dummy context in design mode

Creating a GraphicsContext with null parameters is not a guarantee that
we will get a dummy context that does not call any OpenGL functions. We
need to explicitly define and construct a dummy context that is safe to
use inside the designer.

Affects issue #49

Source/GLControl/DummyGLControl.cs

@@ -37,7 +37,7 @@ namespace OpenTK
 
         public IGraphicsContext CreateContext(int major, int minor, GraphicsContextFlags flags)
         {
-            return new GraphicsContext(null, null);
+            return new DummyContext();
         }
 
         public bool IsIdle
@@ -51,5 +51,121 @@ namespace OpenTK
         }
 
         #endregion
+
+        #region class DummyContext
+
+        class DummyContext : IGraphicsContext, IGraphicsContextInternal
+        {
+            static int instance_count;
+
+            readonly ContextHandle handle = new ContextHandle(new IntPtr(
+                System.Threading.Interlocked.Increment(ref instance_count)));
+
+            IWindowInfo current_window;
+            bool is_disposed;
+            int swap_interval;
+
+            #region IGraphicsContext Members
+
+            public void SwapBuffers()
+            {
+            }
+
+            public void MakeCurrent(IWindowInfo window)
+            {
+                current_window = window;
+            }
+
+            public bool IsCurrent
+            {
+                get { return current_window != null; }
+            }
+
+            public bool IsDisposed
+            {
+                get { return is_disposed; }
+            }
+
+            public bool VSync
+            {
+                get
+                {
+                    return SwapInterval != 0;
+                }
+                set
+                {
+                    SwapInterval = value ? 1 : 0;
+                }
+            }
+
+            public int SwapInterval
+            {
+                get
+                {
+                    return swap_interval;
+                }
+                set
+                {
+                    swap_interval = value;
+                }
+            }
+
+            public void Update(IWindowInfo window)
+            {
+            }
+
+            public GraphicsMode GraphicsMode
+            {
+                get { return GraphicsMode.Default; }
+            }
+
+            public bool ErrorChecking
+            {
+                get
+                {
+                    return false;
+                }
+                set
+                {
+                }
+            }
+
+            public void LoadAll()
+            {
+            }
+
+            public void Dispose()
+            {
+                is_disposed = true;
+            }
+
+            #endregion
+
+            #region IGraphicsContextInternal
+
+            public ContextHandle Context
+            {
+                get { return handle; }
+            }
+
+            public IntPtr GetAddress(IntPtr function)
+            {
+                return IntPtr.Zero;
+            }
+
+            public IntPtr GetAddress(string function)
+            {
+                return IntPtr.Zero;
+            }
+
+            public IGraphicsContext Implementation
+            {
+                get { return this; }
+            }
+
+            #endregion
+        }
+
+        #endregion
     }
 }