srb2::Draw::Chain: make copy assignment private, make bruh moments a little more obvious

If you define a variable with type auto and initialize it
with the result of a builder chain -- e.g.

   auto var = Draw().x(0)

-- the resulting variable will be an instance of
srb2::Draw::Chain, and calling builder methods on it will
modify the instance itself, instead of creating a
temporary, like an instance of srb2::Draw would.

Hiding the copy assignment can make this a little more
obvious by emitting an error if you try to reassign the
variable later. E.g.

    var = var.x(0)

That's the best I can do to mitigate this.

src/v_draw.hpp

@@ -148,6 +148,7 @@ public:
 		constexpr Chain() {}
 		explicit Chain(float x, float y) : x_(x), y_(y) {}
 		Chain(const Chain&) = default;
+		Chain& operator=(const Chain&) = default;
 
 		struct Clipper
 		{