So that textures do not start with random uninitialized contents.

Change-Id: Ie240f5f71ed77d5c6c22a120e96540a2d0d7c2ed
Reviewed-on: https://gerrit.libreoffice.org/70773
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>