New feature optionally to accelerate auto-repeated scrolling.

src/xdisp.c: Remove "static" from declaration of
Qfontification_functions.

src/window.c (window_scroll): bind fontification-functions to nil when
scrolling by whole screens and fast-but-imprecise-scrolling is non-nil.
(syms_of_window): New DEFVAR_BOOL fast-but-imprecise-scrolling.

src/lisp.h (bool): Declare Qfontification_functions extern.

lisp/cus-start.el (all): Add fast-but-imprecise-scrolling.

doc/emacs/display.texi (Scrolling): fast-but-imprecise-scrolling.
Describe new variable.

etc/NEWS: Add entry for fast-but-imprecise-scrolling.