For the Xorg stabilization for July 2016 (bug #579266) we also need to stabilize =x11-libs/libva-vdpau-driver-0.7.4-r4 which drops support for nouveau, which is built in mesa starting from version 12, see bug #588392. Without it, you will get a blocker between mesa-12 and libva-vdpau-driver, because mesa has vaapi? ( >=x11-libs/libva-1.6.0:=[${MULTILIB_USEDEP}] video_cards_nouveau? ( !<=x11-libs/libva-vdpau-driver-0.7.4-r3 ) )
Obviously, this only affects systems with VIDEO_CARDS="nouveau" and the architectures amd64/x86.
amd64/x86 stable