touchpad: fine tune size-based thumb detection

In testing on an Apple Magic Trackpad, thumb touches are reliably
detected by being quite large in the major dimension, but around
half the size in the minor dimension.
42 jobs for master in 5 minutes and 40 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Docker Check
passed #20850
arch:rolling@docker-check

00:00:18

passed #20856
arch:rolling@docker-clean

00:00:27

passed #20847
fedora:27@docker-check

00:00:23

passed #20853
fedora:27@docker-clean

00:00:28

passed #20846
fedora:28@docker-check

00:00:23

passed #20852
fedora:28@docker-clean

00:00:28

passed #20851
freebsd:11.2@docker-check

00:00:18

passed #20857
freebsd:11.2@docker-clean

00:00:27

passed #20848
ubuntu:17.10@docker-check

00:00:23

passed #20854
ubuntu:17.10@docker-clean

00:00:27

passed #20849
ubuntu:18.04@docker-check

00:00:23

passed #20855
ubuntu:18.04@docker-clean

00:00:27

 
  Docker Prep
passed #20862
arch:rolling@docker-prep

00:00:17

manual #20868
allowed to fail manual
arch:rolling@force-docker-prep
passed #20859
fedora:27@docker-prep

00:00:17

manual #20865
allowed to fail manual
fedora:27@force-docker-prep
passed #20858
fedora:28@docker-prep

00:00:17

manual #20864
allowed to fail manual
fedora:28@force-docker-prep
passed #20863
freebsd:11.2@docker-prep

00:00:17

manual #20869
allowed to fail manual
freebsd:11.2@force-docker-prep
passed #20860
ubuntu:17.10@docker-prep

00:00:20

manual #20866
allowed to fail manual
ubuntu:17.10@force-docker-prep
passed #20861
ubuntu:18.04@docker-prep

00:00:18

manual #20867
allowed to fail manual
ubuntu:18.04@force-docker-prep
 
  Build
passed #20885
arch:rolling@default-build

00:00:38

passed #20870
fedora:27@default-build

00:00:44

passed #20878
fedora:28@build-no-debuggui

00:00:50

passed #20879
fedora:28@build-no-debuggui-nodeps

00:00:52

passed #20876
fedora:28@build-no-docs

00:00:31

passed #20877
fedora:28@build-no-docs-nodeps

00:00:45

passed #20874
fedora:28@build-no-libwacom

00:00:35

passed #20875
fedora:28@build-no-libwacom-nodeps

00:00:45

passed #20880
fedora:28@build-no-tests

00:00:31

passed #20881
fedora:28@build-no-tests-nodeps

00:00:33

passed #20871
fedora:28@default-build

00:00:40

passed #20872
fedora:28@default-build-release

00:00:42

passed #20873
fedora:28@scan-build

00:02:33

passed #20882
fedora:28@valgrind

00:00:38

passed #20886
freebsd:11.2@default-build

00:00:21

passed #20883
ubuntu:17.10@default-build

00:00:42

passed #20884
ubuntu:18.04@default-build

00:00:41

 
  Deploy
passed #20887
wayland-web

00:00:14