Preliminary attempt to fix horizontal scroll bar dragging with bidi text.

Preliminary attempt to fix horizontal scroll bar dragging with bidi text.

* w32term.c (w32_horizontal_scroll_bar_handle_click): In y part
of emacs_event return length from si.nPage to si.nMax.
* xdisp.c (set_horizontal_scroll_bar): For right-to-left text
interchange start and end of thumb.
* scroll-bar.el (scroll-bar-horizontal-drag-1): Use cdr of
portion-whole for scrolling right-to-left text.