Relative Mouse Mode Now Supported By Mir On SDL2

The Mir back-end inside SDL2 now has support for the relative mouse mode as is important for some games.

With this commit today to SDL by Canonical's Brandon Schaefer, it appears there is now support for relative mouse mode on Mir rather than only the absolute position mode.

This is important for games like first person shooters and flight simulators that rely upon SDL2 for obtaining relative input.

This has been the first addition to Mir's back-end in SDL2 in 2+ months (then again, the Wayland changes aren't too frequent either).