We've long had double precision mouse coordinates on wayland (e.g.
when rotating a window) but with the new scaling we even have it on
X (and, its also in Xinput2), so convert all the internal mouse/device
position getters to use doubles and add new accessors for the
public APIs that take doubles instead of ints.
We now have a proper MASTER/SLAVE input device split, where
the masters are virtual core input devices and we add fake hw
slave devices for the system pointer and real slave devices for
wintab devices.
We also set the proper source_device on the events so you can
tell which device sent it and properly decode the axis info.
