Just the expensive, pixel reading part, not the bitmap creation, which
isn't thread-safe.

Change-Id: I3723d80aa29ee5f2e604800cb2061c59b0b0324d
Reviewed-on: https://gerrit.libreoffice.org/37978Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>