feat(world): draw actors according to their depth

It allow us to draw some sprites always over some others.

Fixes #8

CHANGELOG.md

@@ -29,6 +29,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **world:** Add `Actor.creationID` to keep the order in which actors have been created
 
+- **world:** Add `Actor.depth` to keep the order in which actors are drawn
+
 ### Changed
 
 - **world:** automatize world integration in a scene

gamecore/modules/world/actors/baseactor.lua

@@ -33,6 +33,7 @@ local Timer   = require(cwd .. "utils.timer")
 function BaseActor:new(world, type, isSolid)
   self.type     = type or ""
   self.isSolid  = isSolid or false
+  self.depth    = 0
 
   self:setManagers(world)
   self:initKeys()

gamecore/modules/world/actors/gfx2D.lua

@@ -34,6 +34,7 @@ function GFX:new(world, x, y, spritename)
 
   local duration = self.sprite.clone:getAnimationDuration()
   self:addTimer("destroy", duration)
+  self.depth    = -100
 end
 
 function GFX:timerResponse(name)

gamecore/modules/world/baseworld.lua

@@ -382,6 +382,15 @@ function BaseWorld:drawActors(id)
   else
     actors = self:getVisibleActors(id)
   end
+
+  table.sort(actors, function(a,b)
+    if (a.depth == b.depth) then
+      return a.creationID < b.creationID
+    else
+      return a.depth > b.depth
+    end
+  end)
+
   for i,v in ipairs(actors) do
     v:draw()
   end