Create framework for custom environments

[Lemonymous]

Changes

• Preserve original function Mission.ApplyEnvironmentEffect as Mission.ApplyEnvironmentEffectVanilla
• Preserve original function Mission.IsEnvironmentEffect as Mission.IsEnvironmentEffectVanilla
• Add comments to describe why the mod loader overrides the environment functions
• Add Mission.CustomEnv - a mission table that custom environments can be added to
• Add logic to mark the board for all custom environments
• Add logic to apply environment effects for all custom environments
• (Custom environments will not automatically "plan" next tiles like LiveEnvironment does. This must be handled by the user)

Usage

Creating an instance of an environment:

• The base environment table is called Environment.
• The base attack-environment table is called Env_Attack and is derived from Environment: Env_Attack = Environment:new{}
• RST's lightning environment is called Env_Lightning and is derived from Env_Attack: Env_Lightning = Env_Attack:new{}
• To instance an environment: local env = Env_Lightning:new()
  • And add it to a mission: mission.CustomEnv["unique_env_id"] = env
• Missions have a LiveEnvironment, which it handles automatically, by Starting it and Planning their tiles.
• Custom environments will have to be manually started and planned.

A simple example of adding and starting a custom environment:

local mission = GetCurrentMission()
local env = Env_Lightning:new()

-- We must manually start custom environments, since we can dynamically add/remove them.
-- In general 'Start' just sets up necessary tables that is required by the environment.
env:Start()

-- Add the environment to the mission using a unique identifier, so we can find our environment later,
-- and avoid collisions with other environments added by other mods.
mission.CustomEnv["unique_env_id"] = env

-- Insert a point to 'Locations'. Many (but not all) environments use this to determine marked tiles that
-- should be affected when the environment effect is being processed.
table.insert(env.Locations, Point(0,0))

Minimalistic example mod

Link to a minimalistic modded weapon that can add environment lightning tiles, using the code added by this PR: https://github.com/Lemonymous/ITB-MinimalisticExampleMods/blob/master/mods/custom_environment_weapon/scripts/env_weapon.lua