enable seq mode for `join`

much faster for large TIFFs

see https://github.com/jcupitt/libvips/issues/717

ChangeLog

@@ -12,6 +12,7 @@
 - rename 'disc' as 'memory' and default off
 - add vips_find_trim(), search for non-background areas 
 - remove lcms1 support, it had bitrotted
+- `join` tagged as seq
 
 2/8/17 started 8.5.8
 - fix transparency detection in merge, thanks Haida

libvips/conversion/join.c

@@ -21,6 +21,8 @@
  * 	- cleanups
  * 19/10/11
  * 	- redone as a class
+ * 25/8/17
+ * 	- tag as sequential
  */
 
 /*
@@ -218,6 +220,7 @@ vips_join_class_init( VipsJoinClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *vobject_class = VIPS_OBJECT_CLASS( class );
+	VipsOperationClass *operation_class = VIPS_OPERATION_CLASS( class );
 
 	VIPS_DEBUG_MSG( "vips_join_class_init\n" );
 
@@ -228,6 +231,8 @@ vips_join_class_init( VipsJoinClass *class )
 	vobject_class->description = _( "join a pair of images" );
 	vobject_class->build = vips_join_build;
 
+	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
+
 	VIPS_ARG_IMAGE( class, "in1", 0, 
 		_( "in1" ), 
 		_( "First input image" ),