This is the standard pattern used by the other 3D graphics API.

BDW has slots for these values, but they aren't actually used until
SKL. Even though the documentation for BDW says they must be zero, it
doesn't seem to cause any harm to program them anyway.

The comment above for the 8x sample positions says that the hardware
implements centroid interpolation by picking the centre-most sample
that is inside the primitive. That implies that it might be worthwhile
to pick a pattern that includes 0.5,0.5. However by experimentation
this doesn't seem to actually be the case. With the sample positions
in this patch, if I modify the piglit test below so that it instead
reports the centroid position, it reports 0.492188,0.421875 which
doesn't match any of the positions. If I modify the sample positions
so that they include one at exactly 0.5,0.5 it doesn't help and it
reports another position which is even further from the center for
some reason.

arb_gpu_shader5-interpolateAtSample-different

Kenneth Graunke experimented with some other patterns that have a
higher standard deviation but I think after some discussion it was
decided that it would be better to pick the same pattern as the other
graphics API in case there are games that rely on this pattern.

(Based on a patch by Kenneth Graunke)

Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben at bwidawsk.net>