Track damage (#3351)

This PR partially addresses #3313 by:
- Tracking DCS hit events and storing the unit hit point updates in
state.json
- Adding logic to kill aircraft when the hit points is reduced to 1, as
opposed to the DCS logic of hit points to 0. This behavior allows
Liberation to track deaths to parked aircraft, which are uncontrolled
and seem to have different damage logic in DCS.
- Tracking damage to TheaterGroundObjects across turns and killing the
unit when the unit's hitpoints reduces to 1 or lower.

Intention is to build on this PR by also tracking front line objects and
statics (buildings). However, larger refactoring is required and so
splitting those into a separate PR.

resources/plugins/base/dcs_liberation.lua

@@ -11,6 +11,7 @@ kill_events = {} -- killed units will be added via S_EVENT_KILL
 base_capture_events = {}
 destroyed_objects_positions = {} -- will be added via S_EVENT_DEAD event
 killed_ground_units = {} -- keep track of static ground object deaths
+unit_hit_point_updates = {} -- stores updates to unit hit points, triggered by S_EVENT_HIT
 mission_ended = false
 
 local function ends_with(str, ending)
@@ -41,6 +42,7 @@ function write_state()
         ["mission_ended"] = mission_ended,
         ["destroyed_objects_positions"] = destroyed_objects_positions,
 		["killed_ground_units"] = killed_ground_units,
+		["unit_hit_point_updates"] = unit_hit_point_updates,
     }
     if not json then
         local message = string.format("Unable to save DCS Liberation state to %s, JSON library is not loaded !", _debriefing_file_location)
@@ -146,6 +148,14 @@ write_state_error_handling = function()
     mist.scheduleFunction(write_state_error_handling, {}, timer.getTime() + WRITESTATE_SCHEDULE_IN_SECONDS)
 end
 
+function update_hit_points(event)
+	local update = {}
+	update.name = event.target:getName()
+	update.hit_points = event.target:getLife()
+	unit_hit_point_updates[#unit_hit_point_updates + 1] = update
+	write_state()
+end 
+
 activeWeapons = {}
 local function onEvent(event)
     if event.id == world.event.S_EVENT_CRASH and event.initiator then
@@ -175,6 +185,15 @@ local function onEvent(event)
         destroyed_objects_positions[#destroyed_objects_positions + 1] = destruction
         write_state()
     end
+	
+	if event.id == world.event.S_EVENT_HIT then
+        target_category = event.target:getCategory()
+		if target_category == Object.Category.UNIT then
+			-- check on the health of the target 1 second after as the life value is sometimes not updated
+			-- at the time of the event
+			timer.scheduleFunction(update_hit_points, event, timer.getTime() + 1)
+		end
+	end
 
     if event.id == world.event.S_EVENT_MISSION_END then
         mission_ended = true