[evince] Fix dpi calculation

[Carlos Garcia Campos <carlosgc src gnome org>]

commit db8a118298552769e835c67d29dfe1fa41de698a
Author: Juanjo Marín <juanj marin juntadeandalucia es>
Date:   Sun Feb 21 12:14:25 2010 +0100

    Fix dpi calculation
    
    Fixes bug #608586.

 shell/ev-utils.c |   13 ++++++++-----
 1 files changed, 8 insertions(+), 5 deletions(-)
---
diff --git a/shell/ev-utils.c b/shell/ev-utils.c
index 8cda889..d77e91f 100644
--- a/shell/ev-utils.c
+++ b/shell/ev-utils.c
@@ -307,14 +307,17 @@ gdouble
 get_screen_dpi (GtkWindow *window)
 {
 	GdkScreen *screen;
-	gdouble    xdpi, ydpi;
+	gdouble    dp, di;
 
 	screen = gtk_window_get_screen (window);
 
-	xdpi = 25.4 * gdk_screen_get_width (screen) / gdk_screen_get_width_mm (screen);
-	ydpi = 25.4 * gdk_screen_get_height (screen) / gdk_screen_get_height_mm (screen);
-	
-	return (xdpi + ydpi) / 2.0;
+	/*diagonal in pixels*/
+	dp = hypot (gdk_screen_get_width (screen), gdk_screen_get_height (screen));
+
+	/*diagonal in inches*/
+	di = hypot (gdk_screen_get_width_mm(screen), gdk_screen_get_height_mm (screen)) / 25.4;
+
+	return (dp / di);
 }