Bug 981132 - set up atfork on the main thread. r=bgirard

--HG--
extra : rebase_source : adbbf08a081dac84de6e1c2d001f32c786b38dc5
Bug 981126 - Prevent glxtest child process from executing the normal shutdown sequence. r=bgirard, r=jrmuizel

--HG--
extra : rebase_source : b28d660a9959895f743515f64381e4a5e0bb2c74
Bug 894933 - Cancel dangling compositor tasks. r=jrmuizel
Bug 983272 - Add a default tiling pref for desktop. r=Cwiiis