Update Friday - 2019.05.31

Feature

* level add difficulty setting

* main scene ui add hotkey

* add more enemy skin

Enhancement

* update main scene ui

* prevent enemy spawn on next to entrance portal

* prevent bullet predict player position too aggressive

* enemy remove high light effect for visibility

* use less obstacle

* use random enemy ai

Bugfix

* prevent character sprite through obstacle tile by new SpriteDrawer

* adjust character sprite render layer by y axis

* make enemy marker render layer on top of portal

* make props render layer on top of portal