LuaRendering.draw_rectangle() "filled" field marked as required

[_CodeGreen]

LuaRendering.draw_rectangle() says the "filled" property is required, but I have accidentally omitted it and it still works without.

[Bilka]

That wasn't quite intended, it was meant to be mandatory. It's interpreting nil as false, which accidentally made it optional. But I don't want to break mods by making it properly required now, so I have instead updated the docs and code for the next version to make it clear that it's optional.