Metafiles may have an external header, so once graphic data is read, we
need to set the size explicitly. Otherwise the width of the EMF image in
the bugdoc will be too small.

Change-Id: I2441eda61278b0f4973db5d9aa14618ccd17c397
Reviewed-on: https://gerrit.libreoffice.org/73064
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>