fix vipsthumbnail on very-many-core systems

see:

https://github.com/jcupitt/libvips/issues/160

we were using UNBUFFERED, but on very-many-core systems this can get out
of order enough to make vipspng free the read object early

ChangeLog

@@ -3,6 +3,7 @@
 - fix similarity rotate+scale, thanks Topochicho
 - fix 16-bit PNG save, thanks John
 - fix dzsave date on Windows, thanks John
+- fix vipsthumbnail on many-core systems, thanks James
 
 25/7/14 started 7.40.5
 - fix a race in im_maxpos_avg()

tools/vipsthumbnail.c

@@ -318,8 +318,10 @@ thumbnail_open( VipsObject *process, const char *filename )
 			"loading jpeg with factor %d pre-shrink", 
 			jpegshrink ); 
 
+		/* We can't use UNBUFERRED safely on very-many-core systems.
+		 */
 		if( !(im = vips_image_new_from_file( filename, 
-			"access", VIPS_ACCESS_SEQUENTIAL_UNBUFFERED,
+			"access", VIPS_ACCESS_SEQUENTIAL,
 			"shrink", jpegshrink,
 			NULL )) )
 			return( NULL );
@@ -328,7 +330,7 @@ thumbnail_open( VipsObject *process, const char *filename )
 		/* All other formats.
 		 */
 		if( !(im = vips_image_new_from_file( filename, 
-			"access", VIPS_ACCESS_SEQUENTIAL_UNBUFFERED,
+			"access", VIPS_ACCESS_SEQUENTIAL,
 			NULL )) )
 			return( NULL );
 	}