Tutorial 3 - Loop does not clear GL_DEPTH_BUFFER_BIT

[DrewImm]

https://github.com/opengl-tutorials/ogl/blob/master/tutorial03_matrices/tutorial03.cpp#L100

In the render loop, glClear( GL_COLOR_BUFFER_BIT) is called. This leads to no output, which I was able to fix by changing to glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

[DrewImm]

Pull request: #83