https://bugs.funtoo.org/browse/FL-1924 Pretty much the idea is that mixing fglrx's libGL with mesa EGL/GLES ends up breaking random software, cogl revdeps in particular. The idea is to use the libs from amd's OpenGL ES SDK instead. Funtoo did this through adding the extra package into ati-drivers. I think that a more elegant way of doing this would be a separate package installing the libs to /usr/lib*/opengl/ati... and revdep on that package in ati-drivers.
using separate package for this is sub-optimal, because AMD release their specific EGL/GLES, which is only make sense to use with fglrx. Installing them from within one ebuild looks more efficient. Anyway, fglrx is just dead (upstream aswell) and should be tree-cleaned. This is beyond sane level to maintain this horrible package and do massive backport patches each new kernel release.