Clean up dead sprites properly.

src/entity.lua

@@ -30,7 +30,7 @@ function Entity:damage(amount)
    self.health = math.max(self.health - (amount - self.armor), 0)
 
    if self.health == 0 then
-      self:remove()
+      self:delete()
    end
 end
 
@@ -38,3 +38,8 @@ function Entity:update()
    local collisions = select(3, self:moveWithCollisions(self.x + self.vector.x, self.y + self.vector.y))
    return collisions
 end
+
+-- override this if you create timers
+function Entity:delete()
+   self:remove()
+end