[Feature 2/4] Xephyr with DRI2 extension

[Haitao.feng]

This part of work will enable Xephyr to run xcompmgr.

The logic is when an X clients wants to get a DRI2Drawable, we create a host
peer invisible window, and give the client the peer DRI2Drawable. When
glXSwapbuffer happens, we copy the peer window content into Xpehyr managed
window and damage that window.