Improve skip offset handling, allow to skip full line on no match

e.g. doMatch for regex stuff will use -1 as skipOffset.
But -1 was not handled at all and you matched again and again.

make && make test works

with patch:

Totals: 132 passed, 0 failed, 0 skipped, 0 blacklisted, 2552ms
Finished testing of TestHighlighterTest *

without patch:

Totals: 132 passed, 0 failed, 0 skipped, 0 blacklisted, 3250ms
Finished testing of TestHighlighterTest *

perf shows improvements, too, found during some AbsInt internal profiling

Differential Revision: https://phabricator.kde.org/D15125