This ensures that if the original src had transparent-black in the
border pixels that our downsampled image does as well. Otherwise,
clamp mode causes these nonzero border pixels to produce vertical
/horizontal smears at the edges of the result.
Bug: chromium:1156804
Change-Id: Id2c3a66de29724db2fcc7954abf7f14937cfb76d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343111
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
