emerge -e world

These are the packages that would be merged, in order:

Calculating world dependencies /
!!! Multiple versions within a single package slot have been
!!! pulled into the dependency graph:

('ebuild', '/', 'x11-proto/renderproto-0.9.2', 'merge') pulled in by
  ('ebuild', '/', 'x11-base/xorg-server-1.3.0.0', 'merge')

('ebuild', '/', 'x11-proto/renderproto-0.9.3', 'merge') pulled in by
  ('ebuild', '/', 'x11-libs/cairo-1.4.10', 'merge')
  ('ebuild', '/', 'x11-libs/libXrender-0.9.2', 'merge')

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously. If such a conflict exists in the
dependencies of two different packages, then those packages can not be
installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man page
or refer to the Gentoo Handbook.

!!! Depgraph creation failed.


-----------------
SOLUTION

add the following lines to /var/lib/portage/world

x11-proto/renderproto-0.9.2
x11-libs/libXrender-0.9.2

(or replace the the existing lines with the lines above)

then run

"emerge -e world"