Increase metrics cache maximal size

Increase metrics cache maximal size

Increase the maximal size of the breakString cache (to compute where
to break lines) from 512kB to 10MB. This has a big impact of cache
hits on large file like the example in #12297, which is now 99%. On
this example the time taken by breakString decreases from 33.5us to
2.4us.

The string width cache has been increased fro 512kB to 1MB, but this
does not make such a big difference.

Additionally, comments and variable names have been improved.

Related to bug #12297.