Added "[sheet" to the texture special commands.

"[sheet:WxH:X,Y" assumes the base image is a tilesheet with W*H tiles on it and crops to the tile at position X,Y.
Basically it works like "[verticalframe" but in 2d.
For testing, I combined the four default_chest images into one.
Merge branch 'master' of https://github.com/LUKK3/minetest