Bug 765: use load_uri to load old tab's document
Kalle reported that after commit
5c96d430c98d7b67cf397aac83cfdfa1788aa6df,
ELinks would crash if the document in the old tab was still loading when a
new tab was opened. The problem was that the new session's download.data
pointer was not updated to point to the session as doc_loading_callback
expects.
Instead of just calling render_document_frames, set up the download and
call load_uri.
(cherry picked from commit
d6116ca83ab683f07b1fa5aa4aa4e8e73bcfeabf)