Internal Program Version: libxcb  1.15

The X protocol C-language Binding (XCB) is a replacement for Xlib
featuring a small footprint, latency hiding, direct access to the
protocol, improved threading support, and extensibility.

The Composite extension causes a entire sub-tree of the window
hierarchy to be rendered to an off-screen buffer. Applications can
then take the contents of that buffer and do whatever they like. The
off-screen buffer can be automatically merged into the parent window
or merged by external programs, called compositing managers.

http://xcb.freedesktop.org