According to the Linux OpenGL ABI, glXGetProcAddressARB must be statically exported by libGL. This does *not* hold true for glXGetProcAddress. We must used the ARB version instead.
Furthermore, glx entry points, unlike wgl, do not depend on any specific OpenGL context. This means we can load them in the constructor of the Glx class.
glXGetProcAddress may return a non-null value even if a function is not
supported by the server. We need to check the extension string using
any GLX extensions. Fixes issue http://www.opentk.com/node/3111 "GLX
extension support is not checked correctly".
Furthermore, mode selection is now performed explicitly by the
X11GLContext constructor.
Added OpenGL 3.0 support for GLX contexts.
Added support for creating dummy contexts (useful if you are creating contexts from an external library).
Cleaned up unused and stale code.
Added xml documentation for the ContextHandle.
Made the casts between ContextHandles and IntPtrs explicit.
Updated all ContextHandle consumers to reflect the explicit cast.
