[ci skip]RenderTexture output ‘stretched’ if render texture size is not same as screen size.

Fix for bug where RenderTexture output is ‘stretched’ if render texture size is not same as screen size. 

It seems that _fullRect and _fullviewPort are switched in the CCRenderTexture.cpp. Discussed in more detail here: http://www.cocos2d-x.org/forums/6/topics/45860