4.6 doesn't support `override` keyword, using preprocessor macro to
simply hide it. The compatibility mode must be explicitly enabled,
though, using GCC46_COMPATIBILITY CMake option.
Desktop OpenGL and OpenGL ES 2 support can be switched using CMake
TARGET_GLES option. All functionality not supported in ES is marked in
documentation.
If targetting OpenGL ES, GLES2/gl2.h is included instead of GLEW.
Mesh class now uses VAOs only in desktop OpenGL, in ES the buffers are
bound on each draw call.