neighbour based algorithm for threads

It does seem to affect since sim falls into a "stable" configuration
where cells rarely change quite quickly.  I'm thinking of making a
better colouring algorithm to see if I can get better images.

src/main.c

@@ -17,12 +17,6 @@
 #include "program_iter.h"
 #include "simulation.h"
 
-#define WIDTH  800
-#define HEIGHT 600
-
-// Our grid will be of length sqrt(NUM_PROGRAMS).  This is the same as
-// 1LU<<(NUM_PROGRAMS_POW_2/2).
-static const size_t GRID_WIDTH = 1LU << (NUM_PROGRAMS_POW_2 / 2);
 static const size_t CELL_WIDTH = WIDTH / GRID_WIDTH;
 
 static const char *VALID_OPS = "<>{}-+.,[]";