Downgrade jessie and sid mesa libraries

This is trickier than precise, as `libegl1-mesa` depends on `libwayland0=0.85.0-2`. We manually fetch and install the packages, then hack the dpkg database to change the dependency (we could modify the `.deb` files but that's a bit more complicated and slower as we need to repack the file). Not pretty, but I couldn't find any better way.

To avoid fetching the `.deb` files on every update, I added a `list_needs_update` function in `ubuntu/prepare`. We could move it to `prepare.sh` if there is a need to use it on other releases.

I'd appreciate if somebody could check if it actually works on affected hardware (@DennisLfromGA @peter-gulka @feeddageek: anyone feeling adventurous?), at least kde/gnome targets on `jessie`. `sid` as a bonus.

Tested in `2014-07-11_10-20-09_drinkcat_chroagh_x11-test.tmp-lts-trusty-fix-jessie-sid_35`, matrix here: http://drinkcat.github.io/chroagh/2014-07-11_10-20-09_drinkcat_chroagh_x11-test.tmp-lts-trusty-fix-jessie-sid_35.html .