Consistently round fOffset in SkOffsetImageFilter

Since SkScalarRoundToInt rounds differently depending on the sign of the
value, care must be taken to perform rounding before any potential
change to the sign - like in SkOffsetImageFilter::onFilterNodeBounds.

Bug: chromium:778204
Change-Id: I3debff7565f45022c7b8566662927149850b1bea
Reviewed-on: https://skia-review.googlesource.com/64020
Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Robert Phillips <robertphillips@google.com>