Ensure the extra border added to the downsampled image doesn't filter in
values from outside the source bounds.
Bug: chromium:1174354
Change-Id: I6c62eeaa57ce4e5341eab24985553f87ab0df666
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378322
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
