SplashXPathScanner: Force inlining of addIntersection

The majority of the code in addIntersection can be optimized away for
vertical (x0 == x1) and horizontal (count == 0) segments, thus the inlined
code is less than the function call setup alone.
This leaves diagonal segments as the only remaining call site, i.e.
inlining here is a net win as well.

Reduces runtime for #57 (fdo#96728, runsforever-poppler.pdf) from 1442 seconds
to 1239 seconds (86%), and #24 (fdo#78728, surf-types.pdf) from ~ 5.0 seconds
to 4.7 seconds.
4 jobs for optimize_splashxpathscanner in 12 minutes and 26 seconds (queued for 1 minute and 18 seconds)
latest
Status Job ID Name Coverage
  Build
passed #27704
build

00:09:57

passed #27705
build_clang_libcpp

00:10:36

passed #27706
build_gcc_49

00:09:57

passed #27707
build_mingw64_fedora

00:10:04