ScrolledWindow: Handle adjustment change during drag gesture

When a drag gesture is in effect, any change to the adjustment's value
is ignored because it is overwritten next time
`scrolled_window_drag_update_cb` is called. This causes evince to jump
to another page if you zoom in or out during a drag gesture because when
zooming, evince updates the adjustment's `upper` and `value` to match
the document's new size while maintaining which area of the document is
currently visible.

Fix this by updating the drag start coordinates so that the gesture can
continue smoothly from the new location.
Status Job ID Name Coverage
passed #129690


manual #129691
win32 allowed to fail manual