Basic Implementation for vehicle damage world option

This adds a new world option to modify vehicle damage. The purpose is to have a global value to alter initial vehicle health.
Vehicles may be more broken with a value > 1 than usual so players may need to scrounge up vehicle parts to get a working one.
In comparison the No Hope mod modifies most mapgen JSON files to create wrecks instead of vehicles.

No Hope does it by setting values for "number" such as [ 3, 4 ] inside vehicle_spawn.json
The problem of this solution is that whenver a new map is created that uses "place_vehicles" it would need to be modified. This is hard to maintain and changes to the maps need to be backported.

The multiplication inside this commit is only a very basic solution that may be changed at will. There were no considerations towards the distribution of non-damaged, damaged and broken components as well as the set_hp values inside the following function

src/options.cpp

@@ -2315,6 +2315,11 @@ void options_manager::add_options_world_default()
          -1, 18, 6
        );
 
+    add( "VEHICLE_DAMAGE", world_default, translate_marker( "Vehicle damage modifier" ),
+         translate_marker( "A scaling factor that determines how damaged vehicles are." ),
+         0.0, 10.0, 1, 0.1
+       );
+
     add( "SPAWN_DENSITY", world_default, translate_marker( "Spawn rate scaling factor" ),
          translate_marker( "A scaling factor that determines density of monster spawns." ),
          0.0, 50.0, 1.0, 0.1

src/vehicle.cpp

@@ -692,8 +692,9 @@ void vehicle::init_state( int init_veh_fuel, int init_veh_status )
         } else {
             //a bit of initial damage :)
             //clamp 4d8 to the range of [8,20]. 8=broken, 20=undamaged.
-            int broken = 8;
-            int unhurt = 20;
+            const float chance =  get_option<float>( "VEHICLE_DAMAGE" ) ;
+            int broken = 8 * chance;
+            int unhurt = 20 * chance;
             int roll = dice( 4, 8 );
             if( roll < unhurt ) {
                 if( roll <= broken ) {