[X11] Fixed SGI_swap_control query

GLX can return valid function pointer to unsupported functions.
We need to check for extension support in addition to the
function pointer.

Fixes crash on OpenSUSE 13.1 in VirtualBox.

Source/OpenTK/Platform/X11/X11GLContext.cs

@@ -380,7 +380,9 @@ namespace OpenTK.Platform.X11
 
         public override void LoadAll()
         {
-            vsync_supported = Glx.SupportsFunction("glXSwapIntervalSGI");
+            vsync_supported =
+                SupportsExtension(display, currentWindow, "SupportsExtension") &&
+                Glx.SupportsFunction("glXSwapIntervalSGI");
             Debug.Print("Context supports vsync: {0}.", vsync_supported);
 
             base.LoadAll();