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: libinput/libinput#138
1 job for libinput-issue-138 in 1 minute and 49 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed job #25334

00:01:49