Added virtual Update and Render member functions to the GameStateManager
Made the Execute and Initialise member functions in GameStateManager virtual

|SOURCE|
[Common/Game/GameStateManager]
* Execute is virtual to allow for user customisation, Initialise is virtual for
  the same reason
* Execute calls the Update and Render virtual member functions which are user-
  customisable
* Update and Render virtual member functions added
Fixed compilation issue with GLExtender

|SOURCE|
[Common/Renderer/OpenGL/GLExtender]
* Added code from the Linux GLExtender, which was not shared in the Common
  source directory
[Linux/Renderer/OpenGL/GLExtender]
* Removed redundant code, which has been moved to the Common GLExtender