This is what the HW provides us.  If we need integer pixel centers, we
want the state tracker to do the lowering pass so that it gets to optimize
on the subtract.  This is also the shader instructions that the blob is
doing on GLES, and is what Vulkan wants too, as was noted in MR !4172.

shader-db on a630:
total instructions in shared programs: 186689 -> 186168 (-0.28%)
total nops in shared programs: 66253 -> 66139 (-0.17%)
total non-nops in shared programs: 120436 -> 120029 (-0.34%)
total dwords in shared programs: 292192 -> 291168 (-0.35%)
total last-baryf in shared programs: 4810 -> 4734 (-1.58%)
total full in shared programs: 10176 -> 10195 (0.19%)
total constlen in shared programs: 54589 -> 54575 (-0.03%)
total sstall in shared programs: 24582 -> 24802 (0.89%)
total (ss) in shared programs: 3921 -> 3925 (0.10%)
total (sy) in shared programs: 1934 -> 1923 (-0.57%)

Tested-by: Marge Bot <mesa/mesa!4223>
Part-of: <mesa/mesa!4223>