[gnome-photos] Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit df721355cfa373ded1d60de43ff74d19f03dfd39
Author: Debarshi Ray <debarshir gnome org>
Date:   Sun Nov 27 15:06:38 2016 +0100

    Update to latest libgd
    
    GdMainView should forget the last non-range selection when leaving the
    selection mode.

 libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/libgd b/libgd
index 8ad0d8c..a6f2d75 160000
--- a/libgd
+++ b/libgd
@@ -1 +1 @@
-Subproject commit 8ad0d8c63ac7be212c54be4e7fb802c239132239
+Subproject commit a6f2d754203582adedb2a5a896a31ce5b73e05d1