Adding music to AdvMap terrain.

There's also an issue where the new terrain textures, which we apply on top of the old ones, don't erase the old ones – they simply visually "override" them. Therefore, a single cell can have multiple different terrain textures with a density of 255 and the same Priority. In this case, the music of the terrain that was applied **first** is played, meaning it appears earlier in the Ground.bin file.