Ported SDL2 Tic Tac Toe to OpenGL

-Everything is drawn using OpenGL.
-The window is now binded with an opengl context.
-Updated CMakeLists to find and link OpenGL. Also copy shaders
 to build directory.
-Moved Rematch button to its own class.
-Digits have their own OnRender function instead of drawing
 directly in the scoreboard.