In View mode, dragging can only ever pan a zoomed in image. In all other
cases it does nothing. This patch initiates a drag drop operation
instead in those other cases, but keeps the panning functionality.
This works for videos as well as images, and also functions in
comparison mode.
When dragging a document, we check if it is zoomable, and if so whether
any part of it is not visible. In this case, we assume panning is
possible and abort the drag drop operation.
BUG: 386034