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 6 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1153236
build

00:03:54

passed #1153241
build_android

00:01:43

passed #1153237
build_clang_libcpp

00:04:23

passed #1153240
build_clazy

00:06:37

passed #1153239
build_mingw64_fedora

00:02:35

passed #1153238
build_ubuntu_16_04

00:06:08