[libchamplain] Fix the previous commit by adjusting offset based on the viewport size

[Jiří Techet <jiritechet src gnome org>]

commit ce564e0cff5199c9b9ba66a2f05db94d5911c791
Author: Jiří Techet <techet gmail com>
Date:   Wed May 8 01:56:01 2013 +0200

    Fix the previous commit by adjusting offset based on the viewport size

 champlain/champlain-view.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/champlain/champlain-view.c b/champlain/champlain-view.c
index 3bf7979..450bf2e 100644
--- a/champlain/champlain-view.c
+++ b/champlain/champlain-view.c
@@ -2512,8 +2512,8 @@ view_set_zoom_level_at (ChamplainView *view,
 
   /* priv->viewport_x, priv->viewport_y are in int which isn't sufficient precision
    * when multiplied by deltazoom - recalculate them */
-  viewport_x = champlain_map_source_get_x (priv->map_source, priv->zoom_level, priv->longitude);
-  viewport_y = champlain_map_source_get_y (priv->map_source, priv->zoom_level, priv->latitude);
+  viewport_x = champlain_map_source_get_x (priv->map_source, priv->zoom_level, priv->longitude) - 
priv->viewport_width / 2.0;
+  viewport_y = champlain_map_source_get_y (priv->map_source, priv->zoom_level, priv->latitude) - 
priv->viewport_height / 2.0;
   new_x = (viewport_x + offset_x) * deltazoom - offset_x;
   new_y = (viewport_y + offset_y) * deltazoom - offset_y;