* Source/OpenTK/Graphics/GraphicsContext.cs:
* Source/OpenTK/Graphics/IGraphicsContext.cs:
* Source/OpenTK/Platform/MacOS/AglContext.cs:
* Source/OpenTK/Platform/X11/X11GLContext.cs:
* Source/OpenTK/Graphics/GraphicsContextBase.cs:
* Source/OpenTK/Platform/Windows/WinGLContext.cs:
* Source/OpenTK/Platform/Dummy/DummyGLContext.cs: Added IsDisposed
property to IGraphicsContext and implemented in all context classes.
Removed 'bool disposed' fields from these classes in favor of the
new property.
Added GraphicsContextBase, which acts as the foundation of all IGraphicsContext implementations.
Added DesktopGraphicsContext, which acts as the foundation of all desktop (i.e. not ES) IGraphicsContext implementations.
Modified all IGraphicsContext implementations to inherit from GraphicsContextBase and/or DesktopGraphicsContext.