(svn r4486) - NewGRF: Create and use a memory pool to manage sprite groups. This

reduces the amount of house keeping we do and the chance of memory 
leaks.

sprite.h

@@ -118,6 +118,7 @@ typedef struct ResultSpriteGroup {
 } ResultSpriteGroup;
 
 typedef enum SpriteGroupType {
+	SGT_INVALID,
 	SGT_REAL,
 	SGT_DETERMINISTIC,
 	SGT_RANDOMIZED,
@@ -127,7 +128,6 @@ typedef enum SpriteGroupType {
 
 struct SpriteGroup {
 	SpriteGroupType type;
-	byte ref_count;
 
 	union {
 		RealSpriteGroup real;
@@ -160,6 +160,4 @@ SpriteGroup *EvalRandomizedSpriteGroup(const RandomizedSpriteGroup *rsg, byte ra
  * (then they are |ed to @waiting_triggers instead). */
 byte RandomizedSpriteGroupTriggeredBits(const RandomizedSpriteGroup *rsg, byte triggers, byte *waiting_triggers);
 
-void UnloadSpriteGroup(SpriteGroup **group_ptr);
-
 #endif /* SPRITE_H */