Atlas: versions and changes

Map colours, lighting and slope shading now match Xaero's, measured against Xaero's maps over the same test scene by day, at dusk and at night.

• Colours: block colours, terrain depth and water depth follow Xaero's world map. The minimap uses the world map's colours, as Xaero's does.
• Lighting: the map darkens with the time of day and shows block light (torches, lava) at night.
• Slopes: shaded as Xaero's, lit from the north; "Full" is now the default.
• Saving: a map region that can't be written (a full disk, say) is kept and saved again later instead of being lost.

Maps saved by 0.1.0 still load.