Use pre-parsed xml for GtkBuilder resource files

This enables the xml-preparser gresource option which
runs the xml through g_markup_parser_context_record(), and saves
the generated binary format instead of the xml. We then check
for the magic marker in the GtkBuilder code so that this automatically
uses g_markup_parser_context_replay() instead, to avoid any
parsing. The binary format is also smaller.

gtk/gen-gtk-gresources-xml.py

@@ -57,7 +57,7 @@ for f in get_files('gesture', '.symbolic.png'):
 xml += '\n'
 
 for f in get_files('ui', '.ui'):
-  xml += '    <file preprocess=\'xml-stripblanks\'>ui/{0}</file>\n'.format(f)
+  xml += '    <file preprocess=\'xml-stripblanks,xml-preparse\'>ui/{0}</file>\n'.format(f)
 
 xml += '\n'
 

gtk/gtkbuilderparser.c

@@ -1269,8 +1269,19 @@ _gtk_builder_parser_parse_buffer (GtkBuilder   *builder,
                                           G_MARKUP_TREAT_CDATA_AS_TEXT,
                                           &data, NULL);
 
-  if (!g_markup_parse_context_parse (data.ctx, buffer, length, error))
-    goto out;
+  if (buffer[0] == 'G' &&
+      buffer[1] == 'M' &&
+      buffer[2] == 'U' &&
+      buffer[3] == 0)
+    {
+      if (!g_markup_parse_context_replay (data.ctx, buffer, length, error))
+        goto out;
+    }
+  else
+    {
+      if (!g_markup_parse_context_parse (data.ctx, buffer, length, error))
+        goto out;
+    }
 
   _gtk_builder_finish (builder);
   if (_gtk_builder_lookup_failed (builder, error))