INT: added depth buffer and backface culling
INT: fixed the error checking in the ShaderProgram class ==> the validation step should check for GL_VALIDATE_STATUS instead of GL_LINK_STATUS
INT: extracted the Mesh implementation in mesh.cpp
FIX: fixed a potential problem in the move assignment operator in the window class
FIX: fixed potential problems in move assignment operator in several classes
INT: added basic support for uniforms in shaders
INT: transformation works (good up to episode #14)
INT: modified engine to provide t in the update function