Fix ruler-mode text-scaling issues

Fix ruler-mode text-scaling issues

* lisp/ruler-mode.el (ruler-mode-text-scaled-width): New function
for computing scaled text width.
(ruler-mode-text-scaled-window-hscroll)
(ruler-mode-text-scaled-window-width): Compute text scaled
`window-width' value.
(ruler-mode-mouse-grab-any-column, ruler-mode-mouse-add-tab-stop)
(ruler-mode-ruler): Change `window-hscroll' into
`ruler-mode-text-scaled-window-hscroll', and change `window-width'
into `ruler-mode-text-scaled-window-width'.