[gegl] operation: Let the source format go through for passthrough nodes

[Debarshi Ray <debarshir src gnome org>]

commit 270a3da33550bf5b252b9df128df56a961f47ed2
Author: Debarshi Ray <debarshir gnome org>
Date:   Fri Apr 8 18:04:37 2016 +0200

    operation: Let the source format go through for passthrough nodes
    
    https://bugzilla.gnome.org/show_bug.cgi?id=764797

 gegl/operation/gegl-operation.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/gegl/operation/gegl-operation.c b/gegl/operation/gegl-operation.c
index 35bf501..b90ef76 100644
--- a/gegl/operation/gegl-operation.c
+++ b/gegl/operation/gegl-operation.c
@@ -310,7 +310,13 @@ gegl_operation_prepare (GeglOperation *self)
   g_return_if_fail (GEGL_IS_OPERATION (self));
 
   if (self->node->passthrough)
-    return;
+    {
+      const Babl *format;
+
+      format = gegl_operation_get_source_format (self, "input");
+      gegl_operation_set_format (self, "output", format);
+      return;
+    }
 
   klass = GEGL_OPERATION_GET_CLASS (self);