[gdk-pixbuf] jpeg: Restore grayscale image support

[Bastien Nocera <hadess src gnome org>]

commit 6dd89e126a277460faafc1f679db44ccf78446fb
Author: A. Bosch <my+gnomebugs progandy de>
Date:   Thu Jul 20 15:07:13 2017 +0200

    jpeg: Restore grayscale image support
    
    Commit c2a40a92fe3df4111ed9da51fe3368c079b86926 restricts the jpeg
    loader to files with 3 or 4 color components.
    
    Also allow JCS_GRAYSCALE images with only 1 color component.
    These grayscale images are supported through explode_gray_into_buf.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785171

 gdk-pixbuf/io-jpeg.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gdk-pixbuf/io-jpeg.c b/gdk-pixbuf/io-jpeg.c
index f59676f..23284a7 100644
--- a/gdk-pixbuf/io-jpeg.c
+++ b/gdk-pixbuf/io-jpeg.c
@@ -1090,6 +1090,9 @@ gdk_pixbuf__jpeg_image_load_increment (gpointer data,
                                has_alpha = FALSE;
                        } else if (cinfo->output_components == 4) {
                                has_alpha = TRUE;
+                       } else if (cinfo->output_components == 1 &&
+                                  cinfo->out_color_space == JCS_GRAYSCALE) {
+                               has_alpha = FALSE;
                        } else {
                                g_set_error (error,
                                             GDK_PIXBUF_ERROR,