glsl: always enable transform feedback mode when xfb_stride defined

glsl: always enable transform feedback mode when xfb_stride defined

This enables in shader defined transform feedback mode even if the
only place xfb_stride is defined is on the global out.

We don't worry about xfb_buffer since Issue 22 c) in the spec says:

   "If the shader has an "xfb_buffer" qualifier identifying a buffer,
    but doesn't declare "xfb_offset" on anything associated with it,
    what happens?

    ...

    variables not qualified with "xfb_offset" are not captured, which
    makes the associated "xfb_buffer" qualifier irrelevant."

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