SplashXPathScanner: Optionally use small_vector from boost

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
6 jobs from optimize_splashxpathscanner2 in 14 minutes and 24 seconds (queued for 5 minutes and 11 seconds)
Status Job ID Name Coverage
  Build
passed #109153
build

00:05:42

passed #109158
build_android

00:01:19

passed #109154
build_clang_libcpp

00:05:39

passed #109157
build_clazy

00:04:17

passed #109155
build_gcc_49

00:08:28

passed #109156
build_mingw64_fedora

00:03:43