ref: add ref_null renderer that does nothing

This was sitting in my git stash for months. Let's publish it now.

ref/null/wscript

@@ -0,0 +1,20 @@
+#! /usr/bin/env python
+# encoding: utf-8
+# mittorn, 2018
+
+def options(opt):
+	pass
+
+def configure(conf):
+	pass
+
+def build(bld):
+	bld.shlib(
+		source   = 'r_context.c',
+		target   = 'ref_null',
+		features = 'c',
+		defines  = 'REF_DLL',
+		use      = 'engine_includes sdk_includes werror',
+		install_path = bld.env.LIBDIR,
+		subsystem = bld.env.MSVC_SUBSYSTEM
+	)