I got this: >>> Messages generated by process 864 on 2019-04-25 22:56:54 CEST for package media-libs/mesa-18.3.6: QA: preinst QA Notice: Symbolic link /usr/lib32/vdpau/libvdpau_gallium.so points to /usr/lib32/vdpau/libvdpau_gallium.so.1.0.0 which does not exist. QA Notice: Symbolic link /usr/lib64/vdpau/libvdpau_gallium.so points to /usr/lib64/vdpau/libvdpau_gallium.so.1.0.0 which does not exist.
This is fixed in 19.0.3. The 18.3 branch is dead, and this doesn't cause any sort of functional issue, so I'm just going to mark as WONTFIX.