try to fix add-line better.

As it turns out, if your line is in the "wrong direction", the previous
code didn't work. You have to shift by the closest point to the origin.

Author: dbp

src/js/trove/make-image.js

@@ -818,7 +818,7 @@
         var color = unwrapColor(maybeC);
         var img   = unwrapImage(maybeImg);
         var line  = image.makeLineImage(x2 - x1, y2 - y1, color);
-        return makeImage(image.makeOverlayImage(line, "left", "top", -x1, -y1, img, "left", "top"));
+        return makeImage(image.makeOverlayImage(line, "left", "top", -Math.min(x1,x2), -Math.min(y1,y2), img, "left", "top"));
       });
 
       f("scene-line", function(maybeImg, maybeX1, maybeY1, maybeX2, maybeY2, maybeC) {