This reverts commit 6702f261.

Fixes #2035


(cherry picked from commit 57888a32)

In a regression from commit 6cb4a630 ("shell: Don't copy whole files
from remote shares"), on some backends (including ftp and archive),
which don't allow for seeking, evince unsuccessfully attempts to open
documents directly, instead of downloading first.  Keep the existing
logic to presumably still optimize cases where seeking is allowed,
but make sure to first download files where seeking is not possible.

Fixes: https://gitlab.gnome.org/GNOME/evince/-/issues/1971

gvfsd-fuse webdav backend (used by gnome-online-accounts
Nextcloud provider) reports property 'can-seek' as TRUE
while it actually does not support seeking to the end of
a file (G_SEEK_END flag) which is a feature needed by Poppler.

So let's detect that by performing a test G_SEEK_END seek and
checking the result.

Fixes #2037