+further multithreading

I split the grid into 4 quadrants which are managed by their own
thread.  Improves speed and uses more resources correctly.

lib.h

@@ -12,9 +12,10 @@ typedef struct State
   size_t dwidth;
 
   size_t window_len;
-  int multiplier;
+  double multiplier;
 
   bool thread_alive;
+  uint64_t payload;
 } state_t;
 
 bool load_from_file(state_t *state, const char *filepath);