Everything has its own texture file instead of all being clustered together, as part of them unbinding everything from specific IDs to make way for the Mod API. This also means texture packs can no longer break new items/blocks (they will just use the default textures until updated), HD and animated textures are locally supported, etc. The game rebinds these textures as they normally are on load. Texture packs will automatically be unstitched when the new version comes out - this is mainly for texture pack makers to adjust their texture packs ahead of time for the change.