fp-image: Simplify minutiae detection tasks

We can just use a GTask to handle the detection while using the
finish function to process the results to the image, so that it
is more predictable when this happens and it does not depend on
a thread returning.

Also remove data duplication when possible, this class wasn't
fully safe anyway to be used concurrently, so there's no point
to copy data when not needed. Also added the hard constraint to
not proceed with minutiae detection if something else is already
doing this.

At the same time we can mark the task to finish early on
cancellation.

libfprint/fpi-image.h

@@ -68,6 +68,8 @@ struct _FpImage
   guint8    *binarized;
 
   GPtrArray *minutiae;
+
+  gboolean   detection_in_progress;
 };
 
 gint fpi_std_sq_dev (const guint8 *buf,