[gegl] buffer: don't overwrite the soft_format we copied in the commit before

[Michael Natterer <mitch src gnome org>]

commit 0c1f41414754df78f4e27e5da2ef8cc6ac7449b2
Author: Michael Natterer <mitch gimp org>
Date:   Wed Jun 25 18:13:34 2014 +0200

    buffer: don't overwrite the soft_format we copied in the commit before

 gegl/buffer/gegl-buffer.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gegl/buffer/gegl-buffer.c b/gegl/buffer/gegl-buffer.c
index 8ba8bd7..1deb848 100644
--- a/gegl/buffer/gegl-buffer.c
+++ b/gegl/buffer/gegl-buffer.c
@@ -685,9 +685,10 @@ gegl_buffer_constructor (GType                  type,
   buffer->tile_storage = gegl_buffer_tile_storage (buffer);
 
   /* intialize the soft format to be equivalent to the actual
-   * format
+   * format, unless the soft format was copied from a source buffer
    */
-  buffer->soft_format = buffer->format;
+  if (! buffer->soft_format)
+    buffer->soft_format = buffer->format;
 
   g_assert (buffer->tile_width == buffer->tile_storage->tile_width);
   g_assert (buffer->tile_height == buffer->tile_storage->tile_height);