Ship rotation in GUI needs to be interpolated to prevent "snapping".

[Andrew Peterson]

Currently, the ship rotation is calculated based on the current heading of the ship. This can change dramatically between updates and causes the ships to "snap" after the last server update. Interpolating to the new rotation over a few frames should make the ship movement look smoother.

The rate of interpolation should be absolute (rotating 15 degress should take the same time as rotating 180) to make sure the ships don't look like they are "sliding" through space.

[Andrew Peterson]

Ship rotation now interpolates to the target rotation give by the server update. Each interpolation lasts 1 second, regardless of how far it needs to rotate. The 0-360 wrap is handled explicitly.

closes issue 12

→ <<cset 95a7e100ca66>>