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/state.hpp

@@ -29,14 +29,14 @@ namespace cw::state
 
   struct DrawState
   {
-    const State &state;
+    State &state;
     struct Bounds
     {
       cw::node::Node leftmost, rightmost;
       f64 lower_val, upper_val;
     } bounds;
 
-    DrawState(const State &state) : state{state} {};
+    DrawState(State &state) : state{state} {};
 
     void compute_bounds(void);
   };