implement GL20 class, change few GL20 function interface
Merge branch 'master' of https://github.com/aevum/libgdx-cpp
synchronize with libgdx SpriteBatch.cpp VertexBufferObject.cpp and ShaderProgram.cpp
synchronize with original libgdx, fixed glDrawElements call for gl20
remove debug line
Merge pull request #33 from turok/master

Implement GL20 in linux