Improved the diagonal movement adjustment handling.

* hildon/hildon-pannable-area.c,
(hildon_pannable_area_init): Initialized new values to handle the
adjustment position.
(hildon_pannable_area_adjust_value_changed): Check if the values
changed before even call the redraw function.
(hildon_pannable_area_size_allocate),
(hildon_pannable_axis_scroll),
(hildon_pannable_area_scroll),
(hildon_pannable_area_jump_to): Improved the diagonal movement
case.

Fixes: NB#113065 (Image viewer image scrolling updates in X, then
Y)