[librsvg] tests: Mark pixels in diff where only the alpha channel differs

[Benjamin Otte <otte src gnome org>]

commit 79c93aa2006f6491059f66b9ba0eeba05531eeee
Author: Benjamin Otte <otte redhat com>
Date:   Sun Oct 18 23:59:27 2015 +0200

    tests: Mark pixels in diff where only the alpha channel differs

 tests/rsvg-test.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/tests/rsvg-test.c b/tests/rsvg-test.c
index d6f73de..29dae79 100644
--- a/tests/rsvg-test.c
+++ b/tests/rsvg-test.c
@@ -99,6 +99,11 @@ buffer_diff_core (unsigned char *_buf_a,
                }
 
                result.pixels_changed++;
+               if ((diff_pixel & 0x00ffffff) == 0) {
+                   /* alpha only difference, convert to luminance */
+                   guint8 alpha = diff_pixel >> 24;
+                   diff_pixel = alpha * 0x010101;
+               }
                row[x] = diff_pixel;
            } else {
                row[x] = 0;