Add the ability to set custom display resolutions (or fake modes if you like). These affect the logical view area and scaling that appears on screen but do not affect the actual display mode the monitor is set to. So the renderer will scale up/down and letter-box (add black bars) as required. (LP: #1639226 mostly complete)
This now works apparently perfectly (try mirout fakemode ...) with the one caveat that the hardware cursor doesn't match up yet. You can work around that by simply running your server with '--cursor software'. Fixes: https://bugs.launchpad.net/bugs/1639226.