Tweaked the aircraft HUD for performance.

Also added ACM gunsight mode changes. When you lock an airbourne target you get slightly different assistance.

Double tapping the "map" key now toggles the artificial horizon ladder on/off for clearer ACM view

Tweaked the Helm HUD screen

Show several islands, not just nearest
Less CPU use for math funcs

Turrets are no longer static, you can move them with a ground unit, you can even do things like pushing them off a cliff, or push friendly turrets away from their original spawn point, and building another one there

Added some performance tweaks to the needeflish and extended awacs radar lookups.

We now only update the sea OR air radar contacts in a given tick, updates of either are spread out better so they do not periodically bunch up

Also added some basic lua optimisation to the radar lookups by localising the global radar tables and using `dist_sq ^ 0.5` instead of `math.sqrt()`

Also added a "fast_dist_sq()" function to only bother doing the vec2_dist_sq() call if the x coord of the two points is within 24km