lib: re-add partial image flag

And activate perimeter points removal if this flag is set

This flag should be set for aes1610, aesx660, aes2501, aes2550
and upektc_img since these sensors may produce incomplete image.

Fixes: #142

libfprint/fpi-image.h

@@ -37,6 +37,7 @@ typedef enum {
   FPI_IMAGE_V_FLIPPED       = 1 << 0,
   FPI_IMAGE_H_FLIPPED       = 1 << 1,
   FPI_IMAGE_COLORS_INVERTED = 1 << 2,
+  FPI_IMAGE_PARTIAL         = 1 << 3,
 } FpiImageFlags;
 
 /**