(svn r2868) Change the way NewGRFs are loaded: The loading process i no longer bolted onto the normal graphics loading.

This has two major advantages:
- Removal of a maze of global variables and distinction of cases from the sprite loading routines, which weren't directly related to the loading process in the first place
- NewGRF actions no longer occupy sprite slots - for example when using DBSetXL this saves about 2000 slots! (you could regard this as a bug fix)
If i didn't make a major mistake this change should have no negative effect on NewGRF support, please test!

newgrf.h

@@ -39,9 +39,8 @@ struct GRFFile {
 extern int _grffile_count;
 extern GRFFile *_first_grffile;
 
-void InitNewGRFFile(const char *filename, int sprite_offset);
-void DecodeSpecialSprite(const char *filename, int num, int spriteid, int stage);
 
+void LoadNewGRF(uint load_index, uint file_index);
 
 #endif /* NEWGRF_H */