[gthumb] png loader: load the transparency when a color palette is used

[Paolo Bacchilega <paobac src gnome org>]

commit 72460aa77ec6d3a251ed6f5dd6ff4618dd37fcd7
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Mon Jan 27 10:11:48 2020 +0100

    png loader: load the transparency when a color palette is used
    
    When a color palette is used there could be an alpha channel.

 extensions/cairo_io/cairo-image-surface-png.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/extensions/cairo_io/cairo-image-surface-png.c b/extensions/cairo_io/cairo-image-surface-png.c
index 038e092e..8017ef68 100644
--- a/extensions/cairo_io/cairo-image-surface-png.c
+++ b/extensions/cairo_io/cairo-image-surface-png.c
@@ -204,7 +204,7 @@ _cairo_image_surface_create_from_png (GInputStream  *istream,
        }
 
        metadata = _cairo_image_surface_get_metadata (cairo_png_data->surface);
-       _cairo_metadata_set_has_alpha (metadata, (color_type & PNG_COLOR_MASK_ALPHA));
+       _cairo_metadata_set_has_alpha (metadata, (color_type & PNG_COLOR_MASK_ALPHA) || (color_type & 
PNG_COLOR_MASK_PALETTE));
        _cairo_metadata_set_original_size (metadata, width, height);
 
        /* Set the data transformations */