Bushes, improve battle interpreter, fix rotation effect and other fixes

* If the "Allow Other Events to continue" message options is active, all events will now keep doing their move route except the event that is creating the messages.
* If you runned the EasyRPG Player accidentally in the wrong folder (e.g. "C:/"), the player would try to index thousands of files before testing whether the folder is a valid RM2k(3) folder. This has been fixed.
* Fix #310

(bitmap_screen.cpp is a mess!)