fix subsample at the command-line

it had the SEQ flag set, causing confusion with vips_sequential() when
it skipped scanlines on startup

see https://github.com/jcupitt/libvips/issues/370

ChangeLog

@@ -6,6 +6,7 @@
 - fix to "make check" in non-C locales [felixbuenemann]
 - use builtin isnan isinf when possible [Lovell Fuller]
 - tune vips_shrinkh(), 30% faster [Lovell Fuller]
+- remove SEQ hint from vips_subsample(), fixes cli performance [erdmann]
 
 1/1/16 started 8.2.1
 - add a compat stub [Benjamin Gilbert]

libvips/conversion/subsample.c

@@ -13,6 +13,8 @@
  * 	- redo as a class
  * 2/11/13
  * 	- add @point to force point sample mode
+ * 22/1/16
+ * 	- remove SEQUENTIAL hint, it confuses vips_sequential()
  */
 
 /*
@@ -265,7 +267,9 @@ vips_subsample_class_init( VipsSubsampleClass *class )
 	vobject_class->description = _( "subsample an image" );
 	vobject_class->build = vips_subsample_build;
 
-	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
+	/* We don't work well as sequential: we can easily skip the first few
+	 * scanlines, and that confuses vips_sequential().
+	 */
 
 	VIPS_ARG_IMAGE( class, "input", 0, 
 		_( "Input" ),