render: add additive_cullback test

https://github.com/w23/xash3d-fwgs/issues/665

render/rendertest.py

@@ -16,7 +16,7 @@ saves = [
 	'brush_02',
 	'c0a0d_emissive',
 	'light_01',
-	#'additive_cullback',
+	'additive_cullback',
 ]
 
 displays = {
@@ -163,18 +163,25 @@ def command_run():
 	copy_assets()
 	render()
 	compare()
-
 	#report()
 
+def command_render():
+	compile()
+	copy_assets()
+	render()
+
 match args.command:
 	case 'run':
 		command_run()
 	case 'compare':
 		command_compare()
+	case 'render':
+		command_render()
 	case 'png':
 		command_png()
 	case _:
 		raise Exception(f'Unknown command {args.command}')
 
 # TODO:
+# - settings object to pass as an argument
 # - HTML report