Avoid weird behavior with large horizontal scrolls.

Avoid weird behavior with large horizontal scrolls.

Without this change, for example, large hscroll values would
mess up Emacs's display on Fedora 15 x86, presumably due to
overflows in int calculations in the display code.
Also, if buffers had long lines, Emacs would freeze.
* window.c (HSCROLL_MAX): Reduce to 100000, and make it visible to GDB.
(set_window_hscroll): New function, containing the old guts of
Fset_window_hscroll.  Return the clipped value.
(Fset_window_hscroll, Fscroll_left, Fscroll_right): Use it.
This avoids the need to check against PTRDIFF_MAX.