fdo#48932: Prefer interpolation-based image rescaling in PDF export

[Gökçen Eraslan]

Hello,

Attached patch changes preferred image resampling method used in PDF
export to BMP_SCALE_INTERPOLATE. That minimizes Moire artifacts in the
images. I want to thank Kendy for his support.

Here is a screenshot: http://imagebin.org/213821

Left one is the default BMP_SCALE_FAST and right one is
BMP_SCALE_INTERPOLATE.

-- 
Gökçen Eraslan