Fixes crash when GL context is changed after init
Fixes crash when GL context is changed after init

Sets/clears the context on start/stopAnimating to prevent 
EXEC_BAD_ACCESS crash when other GL code is invoked (and sets
its own context) after init.
emodeqidao/XBPageCurl,emodeqidao/XBPageCurl,xissburg/XBPageCurl,xissburg/XBPageCurl