gio: Bump copy buffer size to 256k by default

This is small enough that it shouldn't cause problems on most machines
we support, but big enough to increase throughput on a lot of devices
and network protocols.

Note that the actual value is 256k minus malloc overhead, so that it
fits nicely in a 256k block (as suggested by Alexander Larsson).

See https://bugzilla.gnome.org/show_bug.cgi?id=773632

https://bugzilla.gnome.org/show_bug.cgi?id=773823

gio/gfile.c

@@ -2765,7 +2765,8 @@ g_file_copy_attributes (GFile           *source,
   return res;
 }
 
-#define STREAM_BUFFER_SIZE (1024*64)
+/* 256k minus malloc overhead */
+#define STREAM_BUFFER_SIZE (1024*256 - 2 *sizeof(gpointer))
 
 static gboolean
 copy_stream_with_progress (GInputStream           *in,