Fix SkComposeImageFilter's bounds computation and offset handling

This makes SkComposeImageFilter::computeFastBounds compose its
filters' bounds (rather than falling back to
SkImageFilter::computeFastBounds, which takes the union of the bounds).

This also makes SkComposeImageFilter::onFilterImage correctly handle
an offset produced when applying the inner filter; such offsets were
previously ignored.

BUG=chromium:453924

Review URL: https://codereview.chromium.org/908273002
3 files changed