Fix add-line overlay

The origin for the line should be at x1, y1, but since the arguments to
makeOverlayImage move the _background_ right and down, we need to
reverse both. This will mean that the background goes left and up and
the start of the line is at the right place.

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", 0, 0, img, "left", "top"));
+        return makeImage(image.makeOverlayImage(line, "left", "top", -x1, -y1, img, "left", "top"));
       });
 
       f("scene-line", function(maybeImg, maybeX1, maybeY1, maybeX2, maybeY2, maybeC) {