Beta upgrades: background details etc

Numerous changes including:

Press SPACEBAR to pause/unpause. This will freeze everything, including
the cursor.

Dressage manager added. This holds some "ships" that are little more
than patterns of dust set to meander across the screen.

New background added, using debris assets and lots of iterations to make
it look right. Large star distribution tweaked, nebulae carefully
coloured to fit the enemy ships.

Normal maps for puff effects added. These are used in the exhaust trails
of missiles, which have been tweaked to be slow and leave minimal
exhaust to not cause slowdown.

Particles can now set a streak length multiplier.

Emitters can now set a minimum age (0-1).

Ships now have hit points; they take some amount of damage to destroy,
and it regenerates slowly.

Altered lightStencil asset to give correct point illumination on curved
surfaces.