Allow configuration of unit view distance

[Philiquaz]

Adds configuration of unit visibility ranges.
Adds configuration of terrain visibility behaviours.
Supports fortification bonuses, euclidean radii, the sea being flat, and all the bells and whistles.
The default configuration should match vanilla, but on a basis of caution everything is toggled under 1 flag. There is also a performance reason for this, as at a high visibility with hundreds of units around this can become a taxing system.
Resolves #25

This was tested on the Steam version of the game.
Exe offsets should be correct for GoG as there is where most RE took place, but a test should be performed.

OFFSETS FOR PCG.DE ARE UNCHECKED, AS IS THE GAME.
UPDATE BEFORE MERGE.