restart or new game blank board

[redgreenbluee]

(1) when you filled the board and get the game over overlay, clicking new game will remove all the tiles on board but will sometimes fail to initialize, adding two new tiles.

(2) similar to previous problem, pressing restart in the beginning or middle of the game will remove all the tiles on board but will sometimes fail to initialize, adding two new tiles.

-- i think the issue is related to iOS 8 being async with sprite kit run actions. both scenario described above runs the same block of code on M2GameManager.m

[danqing]

The issue is that the display link is sometimes not fired (although added). I'm not exactly sure why it sometimes fail to fire yet. Looking.