vipsthumbnail fix

fix to vipsthumbnail ... --crop could fail for very non-square images,
thanks Alessandro

ChangeLog

@@ -12,6 +12,7 @@
 - vipsthumbnail has --rotate auto-rotate option
 - removed embedded thumbnail reader from vipsthumbnail: embedded thumbnails
   are too unlike the main image
+- fix to vipsthumbnail --crop, thanks Alessandro 
 
 6/3/14 started 7.38.6
 - grey ramp minimum was wrong

tools/vipsthumbnail.c

@@ -199,13 +199,19 @@ calculate_shrink( VipsImage *im, double *residual )
 	int shrink = floor( factor2 );
 
 	if( residual ) {
-		/* Width after int shrink.
+		/* Size after int shrink. We have to try with both axes since
+		 * if they are very different sizes we'll see different
+		 * rounding errors.
 		 */
 		int iwidth = width / shrink;
+		int iheight = height / shrink;
 
 		/* Therefore residual scale factor is.
 		 */
-		*residual = (width / factor) / iwidth; 
+		double hresidual = (width / factor) / iwidth; 
+		double vresidual = (height / factor) / iheight; 
+
+		*residual = VIPS_MAX( hresidual, vresidual ); 
 	}
 
 	return( shrink );