Fix effect mapping computation for displacement-map w/ negative scale

When scale is negative, the displacement will be in the opposite
direction. Hence we need to only use the magnitude of the scale when
computing the effect of the filter.

BUG=641854

Review-Url: https://codereview.chromium.org/2359133003
Cr-Commit-Position: refs/heads/master@{#420396}
5 files changed