Upstream bug: http://bugs.freedesktop.org/show_bug.cgi?id=39645 This affects KDE 4.7.0 when KWin is built using USE="gles -opengl". It immediately crashes when trying to use compositing. Basically, upstream considers this a configuration problem because the ebuild uses: --with-egl-platforms=drm,x11 while it should be using: --with-egl-platforms=x11,drm instead ("x11" should appear before "drm".)
Thanks for the report. Fixed mesa-7.11_rc{2,4} and mesa-9999 in the x11 overlay.