A 2D or 3D graphics library and a GUI library are two quite different
beasts. My GUI library currently uses SDL as a back-end but there is no
reason why it couldn't use a different back-end such as N3888 as long as
it provides similar functionality to SDL.
As far as standardizing 3D in C++ is concerned I don't think it is
warranted as OpenGL is perfectly adequate; people seemed to be obsessed
with abstracting a C API into a C++ one for no good reason: nothing
wrong with C APIs.
/Flibble