[Feature 3/4] Xephyr with DRI2 extension

[Haitao.feng]

This part of work will enable Xephyr to run mutter.

The logic is that I disabled the GLX_texture_from_pixmap extension in Xephyr so
that mutter will have to copy the pixmap into texture and draw the texture with
glTexImage2D.

To support GLX_texture_from_pixmap, we have to support the general GLX pixmap
applications. Currently I have not implemented that.