Refactor compositing a bit to conform to modern Rust style
Implement the beginnings of scrolling via dragging.

There are tearing problems and GLUT does not support the mouse wheel.
Still, it's a start!
Reduce the number of messages needed to exchange buffers from 3 to 2
Move the test harnesses into test/.

This is preparation for removing the `servo-` prefixes from the various
Servo components.
Remove the `servo-` prefix from core components.