fixed #1649: Changed return type to const reference for member variable access.
Merge remote-tracking branch 'cocos2d-x/gles20' into iss1649-return-const
fixed #1649: Reverted some invokings of function that returns const reference.
Merge pull request #1838 from dumganhar/iss1649-return-const

fixed #1649: Changed return type to const reference for member variable access.