Fixed the heap-use-after-free issue.

Just standard multithreading stuff; access to the allocator while hot
threads are running means stuff can change underneath us even /during/
a read.  I've mutex locked state for stuff in the drawing domain which
stops this issue.

src/main.cpp

@@ -52,7 +52,7 @@ constexpr u64 clamp_to_width(const DrawState &ds, f64 val)
          (val - ds.bounds.lower_val);
 }
 
-void draw_tree(const DrawState &ds, const State &state)
+void draw_tree(DrawState &ds, State &state)
 {
   // Number line
   DrawLine(0, HEIGHT / 2, WIDTH, HEIGHT / 2, WHITE);
@@ -63,6 +63,7 @@ void draw_tree(const DrawState &ds, const State &state)
   DrawLine(lower_x, LINE_TOP, lower_x, LINE_BOTTOM, WHITE);
   DrawLine(upper_x, LINE_TOP, upper_x, LINE_BOTTOM, WHITE);
 
+  state.mutex.lock();
   std::stack<std::pair<u64, f64>> stack;
   cw::node::Node n = state.allocator.get_val(0);
   stack.push(std::make_pair(0, n.value.norm));
@@ -87,6 +88,7 @@ void draw_tree(const DrawState &ds, const State &state)
       stack.push(std::make_pair(n.right, right.value.norm));
     }
   }
+  state.mutex.unlock();
 }
 
 using Clock = std::chrono::steady_clock;