Where highlighting of a code block got to — enough to continue it later with
byte-identical output, so a streaming block can highlight only its new lines
instead of the whole block again.
Work the highlighter did since the last SyntaxHighlighter::take_stats.
Byte counters measure input re-processed rather than output produced: a
caller that re-highlights a growing block every frame shows up as the whole
block’s size again and again, whatever the output looks like.