Extend [hv]scroll support to per-window properties.

Extend [hv]scroll support to per-window properties.
(image-mode-current-vscroll, image-mode-current-hscroll): Remove.
(image-mode-winprops-alist): New var to replace them.
(image-mode-new-window-functions): New hook.
(image-mode-winprops, image-mode-window-get, image-mode-window-put): New funs.
(image-set-window-vscroll, image-set-window-hscroll): Use them.
Remove the `window' argument, update callers.
(image-mode-reapply-winprops): Rename image-reset-current-vhscroll.
Use the new functions.
(image-mode-reapply-winprops): New fun.
(image-mode): Use it.