Fix MatrixImageFilter computeFastBounds

This CL makes the bounding box returned from SkMatrixImageFilter::computeFastBounds actually contain the final result pixels. This, in turn, fixes the first two rows of the filterfastbounds GM and the SVG web page cited in the bug.

BUG=418417

Review URL: https://codereview.chromium.org/785893004
1 file changed