improvement(boxes): don't export as a texture the shadow canvas

As now the canvas drawing isn't affected by the transform, it should be 
safe to not export it as a texture anymore.

gamecore/modules/world/actors/utils/boxes/parent.lua

@@ -40,7 +40,7 @@ function Box3D:new(owner, w, h, d)
   self.texture = {}
   self:setTopTexture()
   self:setBottomTexture()
-  self.texture.shadows = nil
+  self.texture.shadows = love.graphics.newCanvas(self.w, self.h)
 
   self:register()
 end
@@ -123,19 +123,14 @@ end
 
 function Box3D:redrawShadowCanvas()
   if (self.needRedraw) then
-    local canvas = love.graphics.newCanvas(self.w, self.h)
-    love.graphics.setCanvas( canvas )
+    love.graphics.setCanvas( self.texture.shadows )
+    love.graphics.clear()
     for i,v in ipairs(self.shadowSources) do
       v.actor:drawShadow(v.x, v.y)
     end
 
     love.graphics.setCanvas( )
 
-    local imagedata = canvas:newImageData()
-    self.texture.shadows = love.graphics.newImage( imagedata )
-    imagedata:release()
-    canvas:release()
-
     self.needRedraw = false
   end
 end