rewrite sand ticking

include/globals.h

@@ -199,11 +199,20 @@ typedef struct {
   uint8_t block;
 } BlockChange;
 
+#define UPDATE_BASIC              (1 << 0)
+// the sand at this position will be moved down immediately when this is processed
+#define UPDATE_FALL_SAND          (1 << 1)
+// the sand below this block will fall soon
+#define UPDATE_CHECK_SAND_FALL (1 << 2)
+
+#define UPDATE_NOW (UPDATE_BASIC | UPDATE_CHECK_SAND_FALL)
+
 typedef struct {
+  short update_kind;
   short x;
   short z;
   uint8_t y;
-  uint8_t awaitTicks;
+  uint8_t await_ticks;
 } DeferredBlockUpdate;
 
 #pragma pack(push, 1)