[feature] More robust map detection

[Derpius]

Is your feature request related to a problem? Please describe.
Due to the reliance on the GM:Initialise hook to prevent trying to load the map before game.GetMap() returns valid data, it is no longer possible to load VisTrace outside of autorun.

Describe the solution you'd like
Either checking if GetMap returns invalid data (if it ever returns invalid data, need to test) or using some other method to get the current map that doesn't need to wait for the gamemode to initialise.

Additional context
https://wiki.facepunch.com/gmod/game.GetMap

[Derpius]

Added in c65e3b5