Only copy subrect in DistanceFieldGen setup

Bug: chromium:896987
Change-Id: I22fe46964957b3452a84bc04411d11407ff240f9
Reviewed-on: https://skia-review.googlesource.com/c/166681
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed