Beta upgrades: new sprites, battle systems

Added ability to diagnose renderManager buffers.

Tweaked framerate HUD element to show bars in multiples of 8: a steady
30fps shows as 4 bars, whereas it showed a flickering 4th in base 10.

Ship class now handles death properly when slaves are attached (they
also explode and report their own death).

Ship navMode AVOID now works better, using new getNearestNeighbourTo()
method in shipManager to avoid situations where the ship would try to
run away from itself and allowing much closer collision checks.

Cloaking systems now run for non-cloaking vessels, in case they were
cloaked by other effects and need to decloak.

Numerous tweaks to thruster placement.

3D models tweaked to give better rim lighting and sensation of
curvature. Turret geometry added. New sprite assets created at 1024*1024
for superior resolution (old sprites were at 512*512). This seems to
have no effect on framerate, although I expect memory use is sharply
increased.

Added mask sprites. These are currently unused, but would be useful for
emit masks.

Code added to spawn enemy versions of the player gunboat. This is
necessary for combat testing as the Prey-A class hasn't been equipped
with weapons yet.

Subsequently, remembered to turn on invulnerability for player vessel.