Comment 2 for bug 1385143

Right, I appear to have fixed this, at least provisionally.

I'm running an Optimus system, so I purged bumblebee* and bbswitch-dkms as well as removing any mesa or *vdpau* packages. I then installed xserver-xorg-video-intel (it wasn't installed for some reason) and restarted X. I have transparency back now, so I assume hardware rendering is all fine and dandy again.