[PE] Properly handle subpixel accumulation across isolation

For pre-CompositeAfterPaint, we let subpixel accumulation propagate
through isolation to keep consistent with legacy compositing code.

For ComposteAfterPaint, we discard subpixel accumulation at
isolation.

Change-Id: Id9460dbb3c7c89931c38119e75f044b9079f3de8
Reviewed-on: https://chromium-review.googlesource.com/c/1488020
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636642}
2 files changed