The goal is to preserve the precision in the gradients of the edges and
only apply the projection into the final cell location. We also include
the half-subrow offset as spotted by Massimo.
References: https://bugs.freedesktop.org/show_bug.cgi?id=84396
Testcase: coverage-rhombus
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
