[gtk/wip/smcv/transparent-is-transparent] reftest-compare: Treat colour channels as undefined if alpha is zero

[Simon McVittie <smcv src gnome org>]

commit 16b9a30655d658a44bf7de316e76481303a408e8
Author: Simon McVittie <smcv debian org>
Date:   Thu Sep 2 22:34:14 2021 +0100

    reftest-compare: Treat colour channels as undefined if alpha is zero
    
    If the alpha channel is zero, it doesn't matter what the values of the
    red, green and blue channels are: the pixel is still fully transparent.
    On most architectures, fully transparent pixels end up all-zeroes
    (fully transparent black), matching what's in the reference PNG file;
    but on mips*el the blend-difference and blend-normal tests get all-ones
    (fully transparent white) and a test failure.
    
    Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/4227
    Signed-off-by: Simon McVittie <smcv debian org>

 testsuite/reftests/reftest-compare.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/testsuite/reftests/reftest-compare.c b/testsuite/reftests/reftest-compare.c
index 5c682e1591..5bccb697b8 100644
--- a/testsuite/reftests/reftest-compare.c
+++ b/testsuite/reftests/reftest-compare.c
@@ -104,7 +104,12 @@ buffer_diff_core (const guchar *buf_a,
           /* check if the pixels are the same */
           if (row_a[x] == row_b[x])
             continue;
-        
+
+          /* even if they're not literally the same, fully-transparent
+           * pixels are effectively the same regardless of colour */
+          if ((row_a[x] & 0xff000000) == 0 && (row_b[x] & 0xff000000) == 0)
+            continue;
+
           if (diff == NULL)
             {
               diff = cairo_image_surface_create (CAIRO_FORMAT_RGB24,