I have an Nvidia card and use the binary drivers, therefore I have compiled xorg-x11 with USE=minimal, which means I have no Xvfb. The gdk-pixbuf ebuild requires Xvfb via the virtualx.eclass, but it is not present on my system. The package should be able to build without Xvfb or an active X session.
got a patch ?
I honestly don't know what changed but I noticed that I have media-libs/gdk-pixbuf-0.22.0-r5 installed properly, so I would guess it was an eclass change.