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.
If you set GDK_SCALE=2 in the environment then all windows will be
scaled by 2. Its not an ideal solution as it doesn't handle
multi-monitors at different scales, and only affects gtk apps.
But it is a good starting points and will help a lot on HiDPI
laptops.
