St: Fix blur radius computation to correspond to current CSS draft

The next draft of the CSS Backgrounds and Borders module will actually
define when the blur radius means. Fix our code to use that definition
(2 * standard deviation) rather than using the 1.9 * that we extracted
from what Mozilla was doing.

https://bugzilla.gnome.org/show_bug.cgi?id=632506

src/st/st-private.c

@@ -435,11 +435,12 @@ blur_pixels (guchar  *pixels_in,
   guchar *pixels_out;
   float   sigma;
 
-  /* we use an approximation of the sigma - blur radius relationship used
-     in Firefox for doing SVG blurs; see
-     http://mxr.mozilla.org/mozilla-central/source/gfx/thebes/src/gfxBlur.cpp#280
-  */
-  sigma = blur / 1.9;
+  /* The CSS specification defines (or will define) the blur radius as twice
+   * the Gaussian standard deviation. See:
+   *
+   * http://lists.w3.org/Archives/Public/www-style/2010Sep/0002.html
+   */
+  sigma = blur / 2.;
 
   if ((guint) blur == 0)
     {