gdkarray: Add support for GDK_ARRAY_NO_MEMSET

If all your callers already initialize the array element as needed,
then we don't need to memset it to zero first.

This is pretty useful for the snapshot state stack, because due
to the per-node-type data area the elements on the stack are
quite large, but often a lot of it is not used.

gdk/gdkarrayimpl.c

@@ -208,8 +208,10 @@ gdk_array(splice) (GdkArray *self,
         memcpy (gdk_array(index) (self, pos),
                 additions,
                 added * sizeof (_T_));
+#ifndef GDK_ARRAY_NO_MEMSET
       else
         memset (gdk_array(index) (self, pos), 0, added * sizeof (_T_));
+#endif
     }
 
 
@@ -279,5 +281,5 @@ gdk_array(get) (const GdkArray *self,
 #undef GDK_ARRAY_NULL_TERMINATED
 #undef GDK_ARRAY_PREALLOC
 #undef GDK_ARRAY_TYPE_NAME
-
+#undef GDK_ARRAY_NO_MEMSET
 #endif