This is achieved by computing a small 2x2 box blur in the AO pass
taking advantage of quad shading, which allows to halve the size
of the bilateral blur kernel.
Reducing the size of the blur kernel has a side effect to kick Adreno
gpu into direct mode, which apparently is much faster here.
Overall we go from 3.4ms to 2.4ms on Pixel4.
The quality is impacted, but not severely. This probably assumes
GPU that have working derivatives.
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