oops. forgot to cite my source.
Clean up ship drawing / calculations.

Switch from Trig.calculate_xy to Trig.lookup_xy to calculate ship
corners. Math was too error prone with negative values & mod.

Switch from using MathX.mod to Math.max+Math.min for calculating ship
corners. This means that the ship won't stretch across the screen when
it is spanning an edge. It does warp the ship, but even at a very slow
velocity, it is hardly noticeable and looks like the point of the ship
just coming in.

I suppose it will look wonky coming across an edge slowly & backwards.
Don't care.

Retired Trig.calculate_xy.
shortened pew life to 50
Fixed pew leaks.