Basically mirroring the API of Trade::AbstractImporter, as that proved
to be useful. The old crazy openSingleData() and openData(horribleStuff)
are deprecated and will be removed in a future release.
No backwards compatibility is provided for font plugins, these need to
be adapted.
This better reflects that the functions modify a global state instead of
a shader-local state and so rebinding may be necessary (unlike with
uniforms, which get preserved).
The old set*() functions are now inline aliases to the bind*()
functions, are marked as deprecated and will be removed in some future
release.