app: document the new display title option

app/config/gimpconfig-dump.c

@@ -313,6 +313,7 @@ static const gchar display_format_description[] =
 "%W  image width in real-world units\n"
 "%h  image height in pixels\n"
 "%H  image height in real-world units\n"
+"%M  the image size expressed in megapixels\n"
 "%u  unit symbol\n"
 "%U  unit abbreviation\n\n";
 

app/display/gimpdisplayshell-title.c

@@ -324,9 +324,8 @@ gimp_display_shell_format_title (GimpDisplayShell *shell,
 
             case 'M': /* image size in megapixels */
               i += print (title, title_len, i, "%.1f",
-                          (double) gimp_image_get_width (image) *
-                          gimp_image_get_height (image) /
-                          1000000.0);
+                          (gdouble) gimp_image_get_width (image) *
+                          (gdouble) gimp_image_get_height (image) / 1000000.0);
               break;
 
             case 'l': /* number of layers */

docs/gimprc.5.in

@@ -513,6 +513,8 @@ certain % character sequences are recognised and expanded as follows:
 .br
 %H  image height in real-world units
 .br
+%M  the image size expressed in megapixels
+.br
 %u  unit symbol
 .br
 %U  unit abbreviation
@@ -572,6 +574,8 @@ certain % character sequences are recognised and expanded as follows:
 .br
 %H  image height in real-world units
 .br
+%M  the image size expressed in megapixels
+.br
 %u  unit symbol
 .br
 %U  unit abbreviation

etc/gimprc

@@ -403,6 +403,7 @@
 # %W  image width in real-world units
 # %h  image height in pixels
 # %H  image height in real-world units
+# %M  the image size expressed in megapixels
 # %u  unit symbol
 # %U  unit abbreviation
 # 
@@ -435,6 +436,7 @@
 # %W  image width in real-world units
 # %h  image height in pixels
 # %H  image height in real-world units
+# %M  the image size expressed in megapixels
 # %u  unit symbol
 # %U  unit abbreviation
 #