xwayland: Use `double` for `xwl_tablet_tool`

So we do not lose subpixel precision in Xwayland.

Suggested-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Closes: https://gitlab.freedesktop.org/libinput/libinput/issues/138
(cherry picked from commit 734b2d69)