Implement cursor relaying for xephyr, and separate out C code.

Xephyr can now use cursor acceleration, which looks way better.
Cursor images are not cached, so there's potential performance improvements to
be had.

Splitting out the C code (and dynamically embedding it in the prepare script)
allows C code to be a first-class citizen and more easily testable.

Fixes #58.
Add xterm dependency to xephyr, and nail down the proper version with GLX.