[libchamplain] defines: Min/max latitude should be 85.05113

[Jiří Techet <jiritechet src gnome org>]

commit 1549c0332ba16d033b315b5341dc2d26377821f4
Author: Jonas Danielsson <jonas threetimestwo org>
Date:   Sun Aug 16 23:04:28 2015 +0200

    defines: Min/max latitude should be 85.05113
    
    So it seems that the map_source_get_y breaks down when
    given values close to +-90. Some googling brought me to the
    conclusion that we should have a MIN/MAX latitude of
    85.05113, since that is what the Mercator projection
    truncates latitude to.
    
    See https://en.wikipedia/wiki/Mercator_projection for
    more information,

 champlain/champlain-defines.h |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/champlain/champlain-defines.h b/champlain/champlain-defines.h
index d8ad82f..8b6f081 100644
--- a/champlain/champlain-defines.h
+++ b/champlain/champlain-defines.h
@@ -25,8 +25,15 @@
 
 #define CHAMPLAIN_API __attribute__((visibility ("default")))
 
-#define CHAMPLAIN_MIN_LATITUDE   -90.0
-#define CHAMPLAIN_MAX_LATITUDE    90.0
+/*
+ * The ordinate y of the Mercator projection becomes infinite at the poles
+ * and the map must be truncated at some latitude less than ninety degrees.
+ *
+ * Using a square aspect ratio for the map, the maximum latitude shown is
+ * approximately 85.05113 degrees.
+ */
+#define CHAMPLAIN_MIN_LATITUDE   -85.05113
+#define CHAMPLAIN_MAX_LATITUDE    85.05113
 #define CHAMPLAIN_MIN_LONGITUDE -180.0
 #define CHAMPLAIN_MAX_LONGITUDE  180.0