A helpful exercise is to consider the work that a compositor actually does and then try implementing a simple CPU-based one. The main operation you'll need is a

A helpful exercise is to consider the work that a compositor actually does and then try implementing a simple CPU-based one. The main operation you'll need is a blit - which you'll find somewhat difficult to optimize. Hardware-accelerated blitting (OpenGL) is so much faster that it's entirely worth it. The only tradeoff is the complexity of OpenGL, but Mesa or other libraries can abstract all of that away.