[gegl] crop: add comment about propagating in-placeability

[Øyvind Kolås <ok src gnome org>]

commit 78b1ae03cdde37bcf34b0a033b31bc3e40519351
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Nov 12 15:26:10 2017 +0100

    crop: add comment about propagating in-placeability

 operations/core/crop.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/operations/core/crop.c b/operations/core/crop.c
index c76ea2a..488a004 100644
--- a/operations/core/crop.c
+++ b/operations/core/crop.c
@@ -170,6 +170,9 @@ gegl_crop_process (GeglOperation        *operation,
 
       output = gegl_buffer_create_sub_buffer (input, &extent);
 
+      /* propagate forked state, meaning that in-place processing is not possible
+       * due to shared nature of underlying data
+       */
       if (gegl_object_get_has_forked (G_OBJECT (input)))
         gegl_object_set_has_forked (G_OBJECT (output));