Relative size should have priority over image size from image headers.

(cherry picked from commit a628c01e)

Change-Id: I80cce79ff87c7eb359cb292526859435cb85f929
Reviewed-on: https://gerrit.libreoffice.org/73434
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>