bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

By the way, I found at least one other case in which Emacs silently 
refuses to do what the programmer asked it to do.

A recipe:

emacs -Q
M-: (setq max-specpdl-size 100) RET
M-: max-specpdl-size RET ;; this returns 100, as expected
C-x C-f lisp/simple.el RET
M-: max-specpdl-size RET ;; this returns 400, why?
M-: (setq max-specpdl-size 100) RET
M-: max-specpdl-size RET ;; this returns 100 again, as expected
C-v ;; lean on the key until the end of the buffer
M-: max-specpdl-size RET ;; this returns 265, why?