Beta upgrades: further performance tweaks

Enhanced methods for rescaling assets. These now deal with alpha
channels correctly.

Increased enemy population; enemies now include Gunboat hulls, big and
tough enemies.

Enhanced autonomous navigation; ships now avoid one another without
either getting too close or getting permanently stuck in one place.

Player plasma cannons now fire very rapidly.

Removed normal shaded elements from missile trail particles. They
weren't rendering much of anything, and their sheer number was dragging
down performance. Missile trails are now a mix of emissive and warp
elements with higher performance.

Increased alpha on puff assets.

Reduced resolution on light stencil. This boosts performance a little
and has no discernable visual effect.