bug#12867: 24.3.50; easy-to-repro crash involving mode line

[martin rudalics]

> These two cache different information, so making such a cache
> efficient is not very trivial.

They share two important properties: (1) They must be accurate for the
same buffer locations (the window that has to be fontified contains the
`point' whose line-number we want to know) and (2) they are usually
invalidated by the same sets of buffer modifications.

martin