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.
It's pointless to have Corrade with compatibility enabled and Magnum
without, because even if it somewhat compiles, it won't link together.
Moreover, explicitly setting compatibility in Magnum while Corrade has
it already enabled is only maitenance burden.
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.