Add g-i as a subproject

If the version of Meson is high enough, we can use gobject-introspection
as a Meson sub-project.

meson.build

@@ -694,6 +694,13 @@ if graphene_has_sse2 or graphene_has_gcc
   endif
 endif
 
+introspection_enabled = get_option('introspection')
+if introspection_enabled and meson.version().version_compare('>= 0.54.0')
+  gi_dep = dependency('gobject-introspection-1.0', required: true,
+    fallback: ['gobject-introspection', 'girepo_dep'],
+  )
+endif
+
 subdir('gtk/css')
 subdir('gdk')
 subdir('gsk')

subprojects/gobject-introspection.wrap

@@ -0,0 +1,5 @@
+[wrap-git]
+directory=gobject-introspection
+url=https://gitlab.gnome.org/GNOME/gobject-introspection.git
+push-url=ssh://git@gitlab.gnome.org:GNOME/gobject-introspection.git
+revision=master