Existing background image was not displayed correctly due to scaling
(see the description in related Jira task).
This commit increases the thickness of the lines, so that the grid
is displayed correctly
Task-number: QTBUG-89896
Pick-to: 6.0 5.15
Change-Id: Iaa8218a5e9852adbc40715e86862ac3b34e1fbec
Reviewed-by: Paul Wicking <paul.wicking@qt.io>