demo-shell: Quick fix for when an opaque fullscreen surface is on top. Make sure the bypass check doesn't get skipped when we need it most (LP: #1378706)
This is a bit hackish. But the problem and the solution are both confined to DemoCompositor so I think good to have something that at least works until we find a more elegant way to demo occlusion testing of decorations. Fixes: https://bugs.launchpad.net/bugs/1378706.
Approved by Alan Griffiths, Alexandros Frantzis, PS Jenkins bot.