Upgrade world-space coordinate vectors to double-precision (issue #22)

Introduces several bugs, probably.
Start adjusting camera setup to make use of high-precision space

Several more entity types still need to set up cameraRelativePosition on init.
Space dust doesn't work yet.
Fix space dust, particles, demo ships
Fix exhaust plume display.

Display and game engine seems spatially stable out to 10^14 units from the origin (~0.01 LY, or thereabouts) which should be enough for any sensible purposes.