ScrnInfo->pixmapPrivate only existed in order to catch invalid
access to the framebuffer by making the backing data NULL across
the VT switch. This was causing more confusion in the higher
layers during mode setting without any real benefit, so remove
it. v2: Kill ShadowModifyPixmapHeader() as well. (ABI change!)
OBS-URL: https://build.opensuse.org/package/show/X11:XOrg/xorg-x11-server?expand=0&rev=292
