I'm not sure which package caused the problem, but all gtk+ apps were broken. I first noticed it when libXCBRender wasn't found. Tried revdep-rebuild, no problems reported. emerge libxcb xcb-util, try revdep-rebuild, multiple ebuilds in list. gtk+ fails with missing pangocairo. emerge pango fails to fix the problem. emerge cairo fails to fix the problem. Then another emerge pango. gtk+ is happy, revdep-rebuild is happy. Not sure of the cause of the problem, nor the solution. Probably can't recreate the situation as I don't know what caused it. Reproducible: Didn't try
Bug 169574 has a better description of the issue actually. *** This bug has been marked as a duplicate of bug 169574 ***