glsl: sort xfb varyings in offset/buffer order
glsl: sort xfb varyings in offset/buffer order

The existing transform feedback code expects to receive the list
of varyings in increasing buffer order.

Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler