Use Lanczos filter and slightly sharpen images for better quality resizing

The default ImageMagick / GraphicsMagick resize filter does not look that good, compared to filters like Lanczos (this is why e.g. Flickr also uses high quality filter to scale down images).

This patch makes these backends use the `resizeImage` method instead of `scaleImage`, have them use the Lanczos filter and sharpen them a little bit. This makes thumbnails and scaled down versions look much better. I'll try to upload screenshots for comparison.