Document the implementation of saveLayer().

The implementation is simple but tricky. Leave explanations to my
future self so that I don't invent a time machine to come back to
now and slap my self.

This change also simplifies the way the GL blending function is
chosen when compositing a layer. It reuses existing OpenGLRenderer
APIs and is easier to understand.

Change-Id: I1b9cf8c5d51e09836d85b8cf157a1c284aa65c59