Arthur: Unflip images vertically

On Linux, images that get handed to the ArthurOutputDev are flipped
vertically for some reason, and that is currently corrected for.  It got
reported in poppler/poppler#850
that on Windows, the images have the correct orientation when they
arrive in ArthurOutputDev.  I have no idea where this difference
comes from.  Anyhow, this patch introduces platform-specific code
to make the final rendered images come out right both on Linux
and Windows.
6 jobs for arthur-unflip-image-on-windows in 4 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3454309
build

00:02:49

passed #3454314
build_android

00:02:18

passed #3454310
build_clang_libcpp

00:04:00

passed #3454313
build_clazy

00:02:31

passed #3454312
build_mingw64_fedora

00:03:32

passed #3454311
build_ubuntu_16_04

00:01:18