Current qt5+cairo uses the plain VCL canvas. This patch is just a
copy of Gtk3Surface (minus comments) with a different update call.
This way the Cairo path now uses the Cairo canvas instead.

It fixes at least tdf#122668 for me, but other Impress bugs might
be fixed this way too.

Change-Id: Iba511c851001753328293c28e53eaa4acc4315d0
Reviewed-on: https://gerrit.libreoffice.org/72921Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
Reviewed-by: Aleksei Nikiforov <darktemplar@basealt.ru>