Upgrade pango to 1.28.3

Fix issue 2656:
some LilyPond builds don't produce font ligatures

Pango 1.28.4 requires glib 2.24.0.
However, GUB's glib is 2.21.5, now.
Therefore, in order to upgrade to newer pango,
it is necessary to also upgrade glib.

gub/specs/pango.py

@@ -12,10 +12,11 @@
     (r'^1\.24', '1.6.0'),
     (r'^1\.26', '1.6.0'),
     (r'^1\.27', '1.6.0'),
+    (r'^1\.28', '1.6.0'),
     ]
 
 class Pango (target.AutoBuild):
-    source = 'http://ftp.gnome.org/pub/GNOME/platform/2.28/2.28.1/sources/pango-1.26.0.tar.gz'
+    source = 'http://ftp.gnome.org/pub/GNOME/sources/pango/1.28/pango-1.28.3.tar.gz'
     patches = ['pango-1.20-substitute-env.patch']
     dependencies = [
             'tools::glib',