Refactor world.c before adding support for isometric margin cropping
Add image struct

This way we only have to calculate an image size once.
Replace some constants

LENGTH means a world/region/chunk dimension that can go in either X or Z
direction.

WIDTH means a horizontal image dimension.

Also added CHUNK_DEPTH which holds the number of pixels below the
surface of a chunk in isometric mode.
Isometric margin cropping seems to work