Throw a GraphicsContextMissingException if GraphicsBindingBase.LoadAll() is called without a current GraphicsContext.
* Graphics/BindingsBase.cs: * Graphics/GraphicsBindingsBase.cs: Moved GraphicsBindingsBase to its own file. Moved BindingsBase to root OpenTK folder (reason: BindingsBase is not specific to OpenTK.Graphics anymore). Documented GetAddress methods.