A function that determines the region to refontify after a change.
This variable is either nil, or is a function that determines the
region to refontify after a change.
It is usually set by the major mode via `font-lock-defaults'.
Font-lock calls this function after each buffer change.
The function is given three parameters, the standard BEG, END, and OLD-LEN
from `after-change-functions'. It should return either a cons of the beginning
and end buffer positions (in that order) of the region to refontify, or nil
(which directs the caller to fontify a default region).
This function should preserve the match-data.
The region it returns may start or end in the middle of a line.