Add wayland.xml to get access to QtCompositor private API

QtCompositor private API needs qwayland-server-wayland.h but this is
generated from wayland.xml by qtwayland and never shipped with the
private headers.

Issue: #82
Plain compositor root item

We used to draw the root item with a black rectangle to do a fade in
effect, but this is not the best way to do it.
Specify precision in contrast effect shader

Looks like Mesa 9.2 demands we always specify precision in shaders.

Issue: #82
Handle shell windows

Issue: #82