* Graphics/ES11/Helper.cs:
* Graphics/ES20/Helper.cs:
* Graphics/BindingsBase.cs:
* Platform/X11/GlxHelper.cs:
* Graphics/OpenGL/GLHelper.cs:
* Platform/X11/X11GLContext.cs: Modified BindingsBase to define
abstract GetAddress method (reason: removes OpenTK.Graphics-specific
code from BindingsBase and allows it to be used in different
bindings).
Implemented GraphicsBindingsBase and modified the OpenGL and OpenGL|ES
bindings to use this.
Modified the GLX bindings to inherit from BindingsBase (reason:
reduces code duplication for extension loading).