GLState: Constify

src/glstate.cpp

@@ -43,7 +43,7 @@ void GLScissorBox::apply(const IntRect &value)
 
 void GLScissorBox::setIntersect(const IntRect &value)
 {
-	IntRect &current = get();
+	const IntRect &current = get();
 
 	SDL_Rect r1 = { current.x, current.y, current.w, current.h };
 	SDL_Rect r2 = { value.x,   value.y,   value.w,   value.h };

src/glstate.h

@@ -43,7 +43,7 @@ struct GLProperty
 
 	void push() { stack.push(current); }
 	void pop()  { set(stack.top()); stack.pop(); }
-	T &get()    { return current; }
+	const T &get()    { return current; }
 	void set(const T &value)
 	{
 		if (value == current)