If you recently un-installed fglrx then switched back to the open source radeon driver you may be left with a couple of broken links to a missing libglx.so and no OpenGL support in X:

[ 65311.851] (II) LoadModule: "glx"
[ 65311.851] (WW) Warning, couldn't open module glx
[ 65311.851] (II) UnloadModule: "glx"
[ 65311.851] (II) Unloading glx
[ 65311.851] (EE) Failed to load module "glx" (module does not exist, 0)

It is necessary to rebuild/reinstall xorg-server:

# emerge xorg-server