thumbnail could fail on HEIC in some modes

in locked UP or DOWN mode, vips_thumbnail() could incorrectly select the
thumbnail from a HEIC image even when it was not large enough to
generate the output image.

Thanks ZorinArsenij

See https://github.com/libvips/libvips/issues/1614

ChangeLog

@@ -5,6 +5,8 @@
 - add fuzz corpus to dist
 - detect read errors correctly in source_sniff
 - fix regression in autorot [malomalo]
+- thumbnail on HEIC images could select the thumbnail incorrectly under some
+  size modes [ZorinArsenij]
 
 20/6/19 started 8.9.1
 - don't use the new source loaders for new_from_file or new_from_buffer, it

libvips/resample/thumbnail.c

@@ -495,15 +495,18 @@ vips_thumbnail_open( VipsThumbnail *thumbnail )
 		/* 'factor' is a gboolean which enables thumbnail load instead
 		 * of image load.
 		 *
-		 * Use the thumbnail if, by using it, we could get a factor >=
-		 * 1.0, ie. we would not need to expand the thumbnail.
+		 * Use the thumbnail if, by using it, we could get a factor >
+		 * 1.0, ie. we would not need to expand the thumbnail. 
+		 *
+		 * Don't use >= since factor can be clipped to 1.0 under some
+		 * resizing modes.
 		 */
 		double shrink_factor = vips_thumbnail_calculate_common_shrink( 
 			thumbnail, 
 			thumbnail->heif_thumbnail_width, 
 			thumbnail->heif_thumbnail_height );
 
-		factor = shrink_factor >= 1.0 ? 1 : 0;
+		factor = shrink_factor > 1.0 ? 1 : 0;
 	}
 
 	g_info( "loading with factor %g pre-shrink", factor );