Custom image resizing

mindtct appears to completely ignore the pixels-per-mm input parameter
(ippmm). When processing AES4000 images, the binarized image is
completely mangled and a lot of ridge information is lost.

Resizing the AES4000's small images results in a huge imaging performance
gain.

We use imagemagick for the resizing, as it's resizing code resamples the
image too (smoothing it out), which further improves performance.

libfprint/fp_internal.h

@@ -126,6 +126,7 @@ struct fp_img_driver {
 	uint16_t flags;
 	int img_width;
 	int img_height;
+	unsigned int enlarge_factor;
 
 	/* Device operations */
 	int (*init)(struct fp_img_dev *dev, unsigned long driver_data);