Everything in Impeller is theoretically treated as if being drawn to
layers of infinite size. Without this change, there's a coverage leak
that occurs in some filters due to the customizable sampling mode. I
should be able to get rid of this 1 pixel padding by refactoring the way
we do sampling in the filter inputs (we already have most of this
infrastructure in place), but this is a correct, safe, and
not-that-expensive solution for the time being.
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