Item textures are padded, despite only block textures needing this treatment

The new anisotropic filtering system in 25w44a applies texture padding to everything on blocks.png-atlas, despite only block textures requiring this.

According to Dinnerbone on Discord, this is undesirable: https://discord.com/channels/1138536747932864532/1424838212702044190/1432774282638921778

This could be fixed by splitting items from the main atlas into their own, dedicated atlas (as was the case from 13w02a to 14w21b).

An example texture atlas from 25w44a is attached - note the unnaturally distended item textures on the right and bottom.