Implement GL20 in linux

I've implemented GL20 in linux backend. I've changed LinuxGraphics class (added to constructor useOpenGl20 flag, added openGL es20 initialization, added swap buffer for opengl20). I've changed "gdxcpp_create_application" function interface added useOpenGLES2 flag.
I've changed VetrexBufferObject.cpp, IndexBufferObject.cpp
I've wrote test for openGL 20 and tested it. I can contribute these test in libgdx-cpp-test