Tickrate can be 0

[MathiasMehl]

The tickrate can be set to 0, this should produce either a warning or an error, as the rendering would be quite boring.

world {
size = 100 [edge], 200 [wrap];
edge = ident;
tickrate = 0;
}

state ident (0, 0, 0) {
become ident;
}