Fix: Tweaked road, water and air depots

[zephyris]

Road and water depots, and airport hangars, had a slightly low offset. This made the water depot not fit well on canal tiles, road depots misaligned from road tiles, etc.

Tweak by lowering offset by one.

Fixes #229

[zephyris]

I've just remembered an extra check I'd like to do - ideally I want to make the position of the source graphics relative to a reference tile in a graphics layer exactly the same as the position relative to the tile. So, might get a minor update!

[zephyris]

Should be pixel-perfect now. Ship depots and airport hangars fill the whole tile, road depots aligned so that their position on the tile exactly matches the position of an imaginary reference tile in the sprite sheet.