Added an OpenGL | ES 2.0 renderer

|SOURCE|
[Common/Window]
* Added a WINDOW_DATA struct to store the EGL and X11 window variables
* Added GetWindowData, which returns a status and the WINDOW_DATA structure
  passed in is filled with the appropriate data from the current window
[Common/EGLWindow]
* Removed temporary OpenGL | ES 2.0 testing code
* Assigned the WINDOW_DATA struct's variables at the end of the Initialise
  member function
[Common/GLES2Renderer]
* Building on the base Renderer, a user-defined clear colour may be used, the
  screen can be cleared, and the front and back buffers swapped
[Common/Game]
* Removed the Window.hpp header, instead using forward-declarations for the
  Window and Renderer member variables
* Added non-specific Renderer calls outside the PreInitialise member function
* Execute calls the Renderer's ClearScreen and SwapBuffers member functions for
  testing the renderer
[Common/Renderer]
* Added an Initialise member function to set the necessary window variables