Proof-of-concept: lazifying measureLineInner

This is a proof-of-concept that does lazy measurement of long lines and partly addresses issue #1022. 
To be more specific, when one requests measurement information regarding char `N`, it 
measures information for characters in `[N-50, N+50]`. The rendering is done in a following fashion:
- tokens that are before `[N-50, N+50]` range are rendered as-is
- tokens that fall inside `[N-50, N+50]` range are splitted into chars

With this patch rendering a single line with 75k characters gets done in `300ms` instead of `16000ms`.

It's not a finished thing and it has couple of unhandled cases; though this effort should obsolete pull request #1609 as it works fast, doesn't require monospaced font and supports (in theory) wrapped lines.

How do you feel about it?