since we only care about appending to this container, and then
traversing it (normally once). So reduce the re-allocations that
std::vector requires

Change-Id: I206a7b82d9eefc1fa3762c4a03e7b5e21136951f
Reviewed-on: https://gerrit.libreoffice.org/30706Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>