[gthumb] cairo utils: always save the has_alpha flag in the metadata

[Paolo Bacchilega <paobac src gnome org>]

commit 3effb1640cd2b53b9910eca531db85bbc22c6904
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sat Apr 20 17:07:11 2019 +0200

    cairo utils: always save the has_alpha flag in the metadata

 gthumb/cairo-utils.c | 38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)
---
diff --git a/gthumb/cairo-utils.c b/gthumb/cairo-utils.c
index 530ffbb0..9b20e682 100644
--- a/gthumb/cairo-utils.c
+++ b/gthumb/cairo-utils.c
@@ -213,6 +213,7 @@ gboolean
 _cairo_image_surface_get_has_alpha (cairo_surface_t *surface)
 {
        cairo_surface_metadata_t *metadata;
+       gboolean                  has_alpha;
        int                       width;
        int                       height;
        int                       row_stride;
@@ -222,31 +223,34 @@ _cairo_image_surface_get_has_alpha (cairo_surface_t *surface)
        if (surface == NULL)
                return FALSE;
 
-       metadata = cairo_surface_get_user_data (surface, &surface_metadata_key);
+       metadata = _cairo_image_surface_get_metadata (surface);
        if ((metadata != NULL) && (metadata->valid_data & _CAIRO_METADATA_FLAG_HAS_ALPHA))
                return metadata->has_alpha;
 
-       if (cairo_image_surface_get_format (surface) != CAIRO_FORMAT_ARGB32)
-               return FALSE;
+       has_alpha = FALSE;
+       if (cairo_image_surface_get_format (surface) == CAIRO_FORMAT_ARGB32) {
+               /* search an alpha value lower than 255 */
 
-       /* search an alpha value lower than 255 */
-
-       width = cairo_image_surface_get_width (surface);
-       height = cairo_image_surface_get_height (surface);
-       row_stride = cairo_image_surface_get_stride (surface);
-       row = _cairo_image_surface_flush_and_get_data (surface);
+               width = cairo_image_surface_get_width (surface);
+               height = cairo_image_surface_get_height (surface);
+               row_stride = cairo_image_surface_get_stride (surface);
+               row = _cairo_image_surface_flush_and_get_data (surface);
 
-       for (h = 0; h < height; h++) {
-               guchar *pixel = row;
-               for (w = 0; w < width; w++) {
-                       if (pixel[CAIRO_ALPHA] < 255)
-                               return TRUE;
-                       pixel += 4;
+               for (h = 0; ! has_alpha && (h < height); h++) {
+                       guchar *pixel = row;
+                       for (w = 0; w < width; w++) {
+                               if (pixel[CAIRO_ALPHA] < 255) {
+                                       has_alpha = TRUE;
+                                       break;
+                               }
+                               pixel += 4;
+                       }
+                       row += row_stride;
                }
-               row += row_stride;
        }
+       _cairo_metadata_set_has_alpha (metadata, has_alpha);
 
-       return FALSE;
+       return has_alpha;
 }