If a player looks at you or a wall you built, you gain charge, and steal
it from them!

Then, building walls now costs 15 charge.

You touching a wall does not do it. This encourages teamwork / using
your walls in clever ways to gain charge.

Also allow 100 walls, and removal of all walls upon death

This reverts commit f41676c672.

I misunderstood the way the camera projections work, and I'm better off
using the original raycasting strategy.

And Camera.project_ray_normal. This is much better

Also, levels now inherit a level object

In multiplayer, if there is only one hero 1, it is ok, but with more
than one, one hero 1 cannot place walls.