Make sure that created empty QImages are filled with transparent pixels.

Copying data from previous QImage on widget resize removes blanking
on window resize when qt5 vcl plugin is used.

Change-Id: If072a4b8b334bc87dbe4aaea9aa8774bb5e202ee
Reviewed-on: https://gerrit.libreoffice.org/63029Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>