3D airport cleanup

I've cleaned up the airport rendering and input a bit more.

The icons are now testing against the 3D depth buffer, so they will now be obscured by the world if they are behind it.

Kind of hard to show what isn't visible haha... But I promise that Australia is behind Europe in this screenshot (simiar to the video in the previous devlog), but now it's airports are properly obscured.

When generating the locations, i did offset airports a little bit above the world, so hopefully we don't end up with any z fighting weirdness.

I've also added a check to the mouse selection code to only allow interaction with airports on the nearest side of the world. It's a pretty simple check, since we are transforming all airport points in 3d around the world, we also transform a point at the center of the earth (0,0,0). And if an airport is closer to the camera than that, we allow clicking on it.