nir/xfb: sort varyings too

nir/xfb: sort varyings too

Right now we are only re-sorting outputs. But it is better to sort too
varyings, as linker expect them to be sorted out (as it was done on
GLSL). For varyings, and to make easier to compute buffer_index, we
sort also by buffer. We could do the same for outputs, but we lack a
reason for that, so we left it as it is (just offset).

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>