Clear previous errors before detaching shaders.
Clear previous errors before detaching shaders.

Otherwise we get a confusing warning.  I think the GL error we're
getting is the one that comes from unbinding our FBO && trying to
tell OpenGL that no FBO is active, but haven't verified.

git-svn-id: 83c4c97093aeea62ac982fbfd516bf9db27f5c99@1569 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok