Currently, each row in the intersections vector is allocated separately,
when the first intersection is added.

To avoid these allocations for common simple polygons,
boost::container::small_vector<4, T> is used, which stores up to
4 intersections inline. small_vector is a header-only class.

For the documents from #57 (fdo#96728) and #24 (fdo#78728), the
runtime/memory is significantly reduced (according to /usr/bin/time -v):
(1) $> pdftoppm -r 18 -aa no runsforever-poppler.pdf
(2) $> pdftoppm surf-types.pdf

Before/After
                                  runsforever-poppler |   surf-types
User time (seconds):                2348.08 / 1773.53 |   7.76 /  5.02
Maximum resident set size (kbytes):   46288 /   45896 |  14076 / 13748