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

Calculating dependencies  ..... ........ ...... ... . .......... done!
[ebuild  N     ] dev-util/itstool-1.2.0  81 kB
[ebuild  N     ] sys-libs/tdb-1.2.11  USE="-python" 471 kB
[ebuild     U ~] dev-libs/glib-2.36.3-r1:2 [2.32.4-r1:2] USE="-debug (-fam) (-selinux) -static-libs -systemtap {-test} -utils -xattr" PYTHON_TARGETS="python2_7%* -python2_5% -python2_6%" 6,472 kB
[ebuild  N     ] x11-themes/gnome-icon-theme-symbolic-3.6.2  208 kB
[ebuild     U ~] x11-libs/cairo-1.12.14-r4 [1.10.2-r3] USE="glib svg -X* (-aqua) -debug -directfb -doc (-drm) (-gallium) (-gles2) -legacy-drivers% -opengl -openvg (-qt4) -static-libs -valgrind% -xcb -xlib-xcb%" 41,145 kB
[ebuild     U ~] media-libs/fontconfig-2.10.92:1.0 [2.8.0-r1:1.0] USE="-doc -static-libs%" ABI_X86="(64%*) -32% (-x32)" 1,490 kB
[ebuild  N     ] gnome-extra/yelp-xsl-3.6.1  576 kB
[ebuild  N     ] app-text/yelp-tools-3.6.1  193 kB
[ebuild     U ~] dev-libs/atk-2.8.0 [2.6.0] USE="introspection nls {-test%}" 620 kB
[ebuild     U ~] x11-libs/pango-1.34.1 [1.30.1] USE="introspection -X* -debug" 992 kB
[ebuild     U ~] x11-libs/gdk-pixbuf-2.28.2:2 [2.26.4:2] USE="X introspection jpeg jpeg2k tiff -debug {-test}" 1,165 kB
[ebuild  N    ~] app-accessibility/at-spi2-core-2.8.0:2  USE="introspection" 427 kB
[ebuild  N    ~] app-accessibility/at-spi2-atk-2.8.1:2  USE="{-test}" 261 kB
[ebuild     U ~] x11-libs/gtk+-3.8.2:3 [3.4.4:3] USE="X introspection vim-syntax (-aqua) -colord -cups -debug -examples (-packagekit) {-test} -wayland -xinerama" 13,381 kB
[ebuild  N    ~] dev-libs/libpeas-1.8.1  USE="gtk python -gjs -glade -seed {-test}" PYTHON_TARGETS="python2_7 python3_2 -python2_6 -python3_3" 519 kB
[ebuild     U ~] dev-libs/totem-pl-parser-3.4.5 [2.32.6-r3] USE="crypt%* introspection quvi -archive {-test%}" 1,571 kB
[ebuild     U ~] media-sound/rhythmbox-2.99.1 [0.12.8-r1] USE="-cdr -clutter% -daap -dbus% -debug% -html% -ipod -libnotify* -libsecret% -lirc -mtp -nsplugin -python {-test} -udev -upnp-av -webkit -zeitgeist% (-gnome-keyring%) (-musicbrainz%)" PYTHON_SINGLE_TARGET="python2_7%*" PYTHON_TARGETS="python2_7%*" 6,227 kB
[blocks B      ] <dev-libs/gobject-introspection-1.36 ("<dev-libs/gobject-introspection-1.36" is blocking dev-libs/glib-2.36.3-r1)

Total: 17 packages (9 upgrades, 8 new), Size of downloads: 75,791 kB
Conflict: 1 block (1 unsatisfied)

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

x11-libs/gdk-pixbuf:2

  (x11-libs/gdk-pixbuf-2.28.2::gentoo, ebuild scheduled for merge) pulled in by
    >=x11-libs/gdk-pixbuf-2.27.1:2[introspection?,X?] required by (x11-libs/gtk+-3.8.2::gentoo, ebuild scheduled for merge)

  (x11-libs/gdk-pixbuf-2.26.4::gentoo, installed) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

x11-libs/cairo:0

  (x11-libs/cairo-1.12.14-r4::gentoo, ebuild scheduled for merge) pulled in by
    >=x11-libs/cairo-1.12.10:=[X?] required by (x11-libs/pango-1.34.1::gentoo, ebuild scheduled for merge)

  (x11-libs/cairo-1.10.2-r3::gentoo, installed) pulled in by
    >=x11-libs/cairo-1.8[X] required by (www-client/firefox-21.0::gentoo, installed)
    >=x11-libs/cairo-1.6:0/0=[X,svg] required by (x11-libs/gtk+-2.24.17::gentoo, installed)
    >=x11-libs/cairo-1.10.0[X] required by (app-office/libreoffice-4.0.4.2::gentoo, installed)
    >=x11-libs/cairo-1.10[X] required by (www-client/firefox-21.0::gentoo, installed)
    >=x11-libs/cairo-1.6:=[X,svg] required by (x11-libs/gtk+-2.24.17::gentoo, installed)
    >=x11-libs/cairo-1.7.6[X?] required by (x11-libs/pango-1.30.1::gentoo, installed)
    >=x11-libs/cairo-1.10.0[aqua?,glib,svg,X?] required by (x11-libs/gtk+-3.8.2::gentoo, ebuild scheduled for merge)

media-libs/fontconfig:1.0

  (media-libs/fontconfig-2.8.0-r1::gentoo, installed) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (media-libs/fontconfig-2.10.92::gentoo, ebuild scheduled for merge) pulled in by
    >=media-libs/fontconfig-2.10.91:1.0= required by (x11-libs/pango-1.34.1::gentoo, ebuild scheduled for merge)

x11-libs/pango:0

  (x11-libs/pango-1.30.1::gentoo, installed) pulled in by
    x11-libs/pango[X] required by (x11-libs/wxGTK-2.8.12.1::gentoo, installed)
    >=x11-libs/pango-1.10.1[X] required by (www-client/firefox-21.0::gentoo, installed)

  (x11-libs/pango-1.34.1::gentoo, ebuild scheduled for merge) pulled in by
    >=x11-libs/pango-1.32.4[introspection?] required by (x11-libs/gtk+-3.8.2::gentoo, ebuild scheduled for merge)

dev-libs/glib:2

  (dev-libs/glib-2.32.4-r1::gentoo, installed) pulled in by
    <dev-libs/glib-2.33:2 required by (dev-libs/gobject-introspection-1.32.1::gentoo, installed)

  (dev-libs/glib-2.36.3-r1::gentoo, ebuild scheduled for merge) pulled in by
    >=dev-libs/glib-2.35.3:2 required by (x11-libs/gtk+-3.8.2::gentoo, ebuild scheduled for merge)
    (and 3 more with the same problem)


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.