[poppler] Getting raster image quality comparable to pdftoppm

[Angus March]

I'm using Poppler to get a Cairo surface and then saving the surface to
a PNG. Unfortunately, the resulting image is of disastrously low
quality. First of all, the resolution is half of what it should be,
gradient backgrounds have these ugly stripes on them, and a background
that should be white is transparent. I expect better from ImageMagick even.
    The executable pdftoppm, on the other hand, yields perfect results.
How do I go about getting a raster image that good through the API?