nir/lower_wpos_center: support adding sample position to fragment coordinate

nir/lower_wpos_center: support adding sample position to fragment coordinate

According to section 14.6 of the Vulkan specification:

   "When sample shading is enabled, the x and y components of FragCoord
    reflect the location of the sample corresponding to the shader
    invocation."

So add a boolean parameter to the lowering pass to select this behavior
when we need it.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>