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.