Mir 0.1.3 isn't a particularly exciting update and most of the changes come down to minor bug-fixes, but there are a few notable items.
In scanning through the Bazaar revision history, one of the notable items for Mir 0.1.3 appear to come down to the GBM backend gaining server-side infrastructure for SHM buffer support, in a later commit the GBM back-end then enabled support for shared memory software buffers using normal system shared memory over Mesa "hardware" buffers. The software shared memory GBM buffers provide better performance to fix some Mir performance regressions.
The only other item sticking out with the Mir development code for 0.1.3 is the re-licensing of the Android graphics and GBM code to the LGPLv3 license from GPLv3.
Other individual Mir 0.1.3 changes can be found via the Bazaar Launchpad repository for Mir.