Bug#712556: More info.
(Me, too!) The problem seems to be that I've got libgbm1 v9.1.3-6 on my system, but libegl1-mesa v8.0.5-6. That is, aptitude managed to upgrade libgbm1 without upgrading the rest of mesa. This seems to have broken my system.
Upgrading libegl1-mesa to v9.1.3-6 (and then using aptitude to ensure that the rest of the related mesa packages are also updated to 9.1.3-6) seems to have fixed the problem. (But gnome-shell is still hosed, have to figure out what's going on there.)
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