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.