Trader airship is in

Added a couple of things, time for a new version:

• Added trader airship with animated trader onboard. The shop is not just a white cube anymore.
• Changed how the surface ambience can be heard underground. Previously volume depended on depth under the surface, now it depends on distance from the nearest hole.
• Fixed a bug that caused sand to become black in some places.
• Fixed the morale display. It's the number in the upper right corner. Now it shows the players morale, that increases with every resource extracted, and decreases when digging ordinary cubes. Morale increases dig speed additionally on top of fulgurite, so the two bonuses can be stacked.
• Added a barrier that prevents the player from leaving the play area. It appears when the player gets close to the edge.