Added an OpenGL-accelerated SDL2 window

|BUILD CONFIGURATION|
[Linux]
* Added SDL2 library to the list of linked libraries

|SOURCE|
[Common/Game]
* Added SDL2 window and OpenGL context member variables
* Initialise initialises the SDL2 window and the OpenGL context
* Execute clears the OpenGL window to read, swaps the buffers, waits 2 seconds,
  then shuts down the window, OpenGL context, and SDL2