[gtk+/gtk-3-14] gtkcairoblur: Minor restructure

[Matthias Clasen <matthiasc src gnome org>]

commit d34f4f6f57c8eed5cf6fc5fd54bab49ddf1ccaa0
Author: Alexander Larsson <alexl redhat com>
Date:   Thu Mar 19 16:25:25 2015 +0100

    gtkcairoblur: Minor restructure
    
    This just moves get_box_filter_size to the top and makes
    it a macro (so it can be used as a constant later).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=746468

 gtk/gtkcairoblur.c |   15 +++++++++++++--
 1 files changed, 13 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkcairoblur.c b/gtk/gtkcairoblur.c
index b3f14db..1d2cb9f 100644
--- a/gtk/gtkcairoblur.c
+++ b/gtk/gtkcairoblur.c
@@ -25,6 +25,19 @@
 #include <math.h>
 #include <string.h>
 
+/*
+ * Gets the size for a single box blur.
+ *
+ * Much of this, the 3 * sqrt(2 * pi) / 4, is the known value for
+ * approximating a Gaussian using box blurs.  This yields quite a good
+ * approximation for a Gaussian.  For more details, see:
+ * http://www.w3.org/TR/SVG11/filters.html#feGaussianBlurElement
+ * https://bugzilla.mozilla.org/show_bug.cgi?id=590039#c19
+ */
+#define GAUSSIAN_SCALE_FACTOR ((3.0 * sqrt(2 * G_PI) / 4))
+
+#define get_box_filter_size(radius) ((int)(GAUSSIAN_SCALE_FACTOR * radius))
+
 /* This applies a single box blur pass to a horizontal range of pixels;
  * since the box blur has the same weight for all pixels, we can
  * implement an efficient sliding window algorithm where we add
@@ -218,8 +231,6 @@ _gtk_cairo_blur_surface (cairo_surface_t* surface,
  * http://www.w3.org/TR/SVG11/filters.html#feGaussianBlurElement
  * https://bugzilla.mozilla.org/show_bug.cgi?id=590039#c19
  */
-#define GAUSSIAN_SCALE_FACTOR ((3.0 * sqrt(2 * G_PI) / 4) * 1.5)
-
 int
 _gtk_cairo_blur_compute_pixels (double radius)
 {