[gegl] Bug 791424 - multithreading slows down color to grey.

[Jehan Pagès <jehanp src gnome org>]

commit 58fcebb96505459c9aa29f910f04315d988e9c0c
Author: Jehan <jehan girinstud io>
Date:   Sun Dec 10 16:22:24 2017 +0100

    Bug 791424 - multithreading slows down color to grey.
    
    This makes the operation unbearably slow though the results don't seem
    wrong. Let's disable multi-threading until we improve this.

 operations/common/c2g.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/operations/common/c2g.c b/operations/common/c2g.c
index 73b647f..4330594 100644
--- a/operations/common/c2g.c
+++ b/operations/common/c2g.c
@@ -375,8 +375,13 @@ gegl_op_class_init (GeglOpClass *klass)
   operation_class = GEGL_OPERATION_CLASS (klass);
   filter_class    = GEGL_OPERATION_FILTER_CLASS (klass);
 
-  filter_class->process    = process;
-  operation_class->prepare = prepare;
+  filter_class->process     = process;
+  operation_class->prepare  = prepare;
+  /* This operation is already quite slow, but it is terrible when
+   * multi-threaded. Disable multi-threading until we optimize it.
+   * See https://bugzilla.gnome.org/show_bug.cgi?id=791424
+   */
+  operation_class->threaded = FALSE;
 
   /* we override defined region to avoid growing the size of what is defined
    * by the filter. This also allows the tricks used to treat alpha==0 pixels