Multiplying padding in Layout object for deterministic tiling.

Without this adjustment, retina and non-retina runs of the Layout
builder would result in different tiling outputs. Multiplying the
requested spacing by the pixel ratio seems to result in the same end
result with only the pixel ratio different, allowing us to not have to
repeat positioning CSS declarations between retina and non-retina images
in the more common case where the same spacing is requested for both