In early March was an early Mesa Mir patch for when Canonical first announced their alternative to using X.Org and Wayland. That patch didn't receive much attention and Canonical didn't put out any patches since that point.
Mir support was not merged for Mesa 9.2 as I pointed out recently even though this is the version of Mesa to be used by Ubuntu 13.10, the first Mir-enabled Linux release. Mesa 9.2 was just branched this week -- meaning no more features for Mesa 9.2 but only stable fixes -- so now Canonical has decided to put out their new Mir patches.
Christopher James Halse Rogers of Canonical posted the set of 15 Mesa patches on Saturday morning. The changes include work on GBM, Gallium DRI Image 7 support, prime-in-DRI buffer support, and then the new Mir EGL platform. Rogers also says he's working on a Radeon kernel DRM patch too as it's needed for how they want to deal with the new Gallium DRI image support. The patch series was posted to mesa-dev but at the moment no one has commented on the series.
The Mir EGL platform itself is about 450 lines of code. With Mesa 9.2 having been branched, the next release to potentially offer this mainline support would be Mesa 9.3 / Mesa 10.0 and that would arrive around November or December given their new Mesa release plans. Ubuntu 13.10 therefore will end up shipping their own patched version of Mesa to offer Mir support. Meanwhile, Wayland support has been integrated into Mesa for many months.