Simple terrain map mode

[mmyers]

Implement a map mode using the terrain_override lists from terrain.txt (EU4).

Example of a terrain definition:

forest = {
    color = { 31 116 16 }

    type = forest
    sound_type = forest

    movement_cost = 1.25
    defence = 1
    combat_width = -0.20
    supply_limit = 4
    local_development_cost = 0.2
    nation_designer_cost_multiplier = 0.9

    terrain_override = { 
        2 4 8 9 11 18 29 30 31 315 4123 4124 # Sweden
        309 1955 #Russia
        2611 996 997 998 2575 #n am

        1054 1046 10
        993 1011 2572 2583 2671

        978 980 2022 2517 2573 2582 2593
        960
        1085 1246
    }                                      
}

Provinces in the terrain_override list have the forest terrain regardless of their color in terrain.bmp.