Macros for treatment of pixels in SDL_Surface.

Need to find best way of getting an int type which is
guaranteed to use exactly 32 bit.

lib/state.h

@@ -3,6 +3,11 @@
 
 #include "settings.h"
 #include <SDL/SDL.h>
+#include <stdint.h>
+
+#define PX_(p, x, y, w, h) ((p)[(x)+(y)*(w)])
+#define PX(i,x,y) (PX_((uint32_t*)(i)->pixels, (x), (y), (i)->w, (i)->h))
+#define iter_pixels(s,x,y) for (x = 0; x < s->w; x++) for (y = 0; y < s->h; y++)
 
 /**
  * Describes the current state of a go board.