Function: font-lock-extend-jit-lock-region-after-change

Function meant for `jit-lock-after-change-extend-region-functions'.
This function does 2 things:
- extend the region so that it not only includes the part that was modified
but also the surrounding text whose highlighting may change as a consequence.
- anticipate (part of) the region extension that will happen later in
`font-lock-default-fontify-region', in order to avoid the need for
double-redisplay in `jit-lock-fontify-now'.