Mark ARB_robustness as done.
This should be strongly typed since forever.
Moved texture and image formats out of the classes.

Advantages:

 * The enums were large (600-800 lines) and they polluted the header,
   now they are in separate files (except for BufferTexture, which has
   the enum small enough to be left in the same file).
 * Image classes now don't need to include OpenGL headers, as they were
   needed only for the enum values. With advantage of C++11's forward
   enum declarations there is no need to include the enum headers
   anywhere in implementation, only when particular values are needed.
 * The values are now less verbose:

    AbstractTexture::InternalFormat::RGB8 // before
    TextureFormat::RGB8                   // now

 * Resolved another "trivial choice" problem (thanks @JanDupal for
   introducing this term to me): how to specify the format if there are
   ten ways to do it (some being massively confusing):

    Image2D::Format f = AbstractImage::Format::RGB; // too long...
    Image2D::Format f = Image3D::Format::RGBA; // why 3D? this works?
    Image2D::Format f = BufferImage1D::Format::RGBA; // wat?

   It is even worse (and more verbose) with textures:

    Texture2D::InternalFormat f =
        CubeMapTextureArray::InternalFormat::RGB8; // this is allowed?

To have consistent naming this change was done also with
BufferTexture::InternalFormat (now BufferTextureFormat), although there
were no trivial choice issues and the enum isn't too large. But at least
it is now less typing.
Updated Doxyfile for Doxygen 1.8.4.

Also disabled some new unwanted features.
Updated external OpenGL headers.
Support for EXT_disjoint_timer_query ES2 extension.
Moved sampler state enums into new Sampler class.

Solves another trivial choice (similar to the one with TextureFormat
etc. in 7de45c98b1aea3ecd530b819d9649d5f5a4628e5), less typing and also
preparation for ARB_sampler_objects extension.