Note about implementation: There is already IVideoDriver::getScreenSize() which is more or less the getter for this function. But I don't think it belongs in IVideoDriver and also this function is about the window and not the screen obviously. So might make sense adding getWindowSize and deprecating getScreenSize, but that needs further changes in GUI. For now I just added a comment and let the rest be as it was.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4693 dfc29bdd-3216-0410-991c-e03cc46cb475