st/theme-node: Support "auto" in lengths

This allows resetting a fixed size to the default in a more specific
selector.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/971

src/st/st-theme-node.c

@@ -1707,7 +1707,13 @@ do_size_property (StThemeNode   *node,
                   CRDeclaration *decl,
                   int           *node_value)
 {
-  get_length_from_term_int (node, decl->value, FALSE, node_value);
+  CRTerm *term = decl->value;
+
+  if (term->type == TERM_IDENT &&
+      strcmp (term->content.str->stryng->str, "auto") == 0)
+    *node_value = -1;
+  else
+    get_length_from_term_int (node, term, FALSE, node_value);
 }
 
 void