(svn r13674) -Fix [FS#2127]: crash when drawing a non-real sprite. The drawing of the non-real sprite is caused when two NewGRFs replace the same sprite and the first replaces it with a real sprite (and thus assumes it remains a real sprite) and the second replaces it with a non-real sprite. OpenTTD already looked at whether the sprite to load should be seen as a real or non-real sprite, but it failed to replace non-real sprites with a substitute real sprite when getting the sprite from the cache.

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