Got rid of InternalFormat class, now it is all in one big enum, as each
version (desktop, ES2, ES3) has different requirements and it can't be
done that way anymore (moreover that was terribly ugly solution).
Some target platforms supply their own OpenGL headers, thus we cannot
use our own from ES 3.0 and compilation fails.
On the other hand, this will be better for users as usage of unsupported
features will be catched right during compilation and not at runtime.
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.