improvement(world): only draw visible actors when a camera is active

It'll make the game less laggy. For the moment, no need to only update 
visible actors.

Fixes #14

gamecore/modules/world/baseworld.lua

@@ -29,6 +29,8 @@ local BaseWorld = Object:extend()
 local Sti           = require(cwd .. "libs.sti")
 local CameraSystem  = require(cwd .. "camera")
 
+local PADDING_VALUE = 10/100
+
 -- INIT FUNCTIONS
 -- All functions to init the world and the map
 
@@ -138,6 +140,17 @@ function BaseWorld:getActors()
   return self.actors
 end
 
+function BaseWorld:getVisibleActors(id)
+  local camx, camy, camw, camh = self.cameras:getViewCoordinate(id)
+  local paddingw  = camw * PADDING_VALUE
+  local paddingh  = camh * PADDING_VALUE
+  local x = camx - paddingw
+  local y = camy - paddingh
+  local w = camw + paddingw * 2
+  local h = camh + paddingh * 2
+  return self:queryRect(x, y, w, h)
+end
+
 -- INFO FUNCTIONS
 -- Give infos about the world
 
@@ -360,7 +373,12 @@ function BaseWorld:draw(dt)
 end
 
 function BaseWorld:drawActors(id)
-  local actors = self:getActors()
+  local actors
+  if (id == nil) then
+    actors = self:getActors()
+  else
+    actors = self:getVisibleActors(id)
+  end
   for i,v in ipairs(actors) do
     v:draw()
   end