Clipboard should "keep last image" by default

Hi,

when taking a sufficiently large screenshot of at least this wine application with xfce4-screenshooter (window or selection) and copying that to the clipboard, pasting, e.g. into libreoffice writer, fails.

Interestingly when the selected window region is small, it works, which smells like a bug to me. When starting xfsettingsd with

XFSETTINGSD_NO_CLIPBOARD=true and using xfce4-clipman with the "copy last image" option checked, it always works.

The same occurs within MATE, which probably has it's own clipboard manager..

I also tested gnome-screenshot - the pasting works, if gnome-screenshot is not closed before pasting (while xfce4-screenshooter closes itself after copying to clipboard, gnome-screenshot remains open).

This seems to be a pointer that the common underlying technology (which is it?) is broken - it looses larger images once the referenced screenshot application closes. However, to have another unreliable backend less, I suggest to copy the last image by default also in the clipboard used within xfsettingsd.

Tested on mostly vanilla Debian Buster.

Btw, if libreoffice-gtk3 is used, it crashes on close, this does not happen in the gtk2-version...

Thanks for the effort

Tycho