ReqIF says the image should be either PNG or has a PNG fallback and then
it can be something else. We used to convert images to PNG to avoid
writing any native data, but results in data loss.

So instead:

1) Write the original image as an RTF fragment, so it's not lost.

2) Some ReqIF parsers expect an RTF fragment even for PNG, so make sure
we always write the RTF fragment, even if that means a small
duplication.

Change-Id: Ida0fcaa58d56b9e11f81992307505599807353b5
Reviewed-on: https://gerrit.libreoffice.org/55430Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins