fdo#72488: Broken text when showing visible space

Turning on showing nonprinting characters replaces the space with bullet
character, but still draws the text with the original kern array, this
works fine until there are ligatures involving the space character as
the number of glyphs after replacing the space with the bullet will be
different and the kern array will be completely off.

This is a hack that gives up on replacing the space with a bullet when
its width is zero, not sure if it would interfere with other legitimate
uses.

Change-Id: I5ed17132ead7cd141a4e8b0372e37541c163be30
Reviewed-on: https://gerrit.libreoffice.org/6995
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>