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Prior to this CL, the entire parent surface was read back to form the
input for the backdrop-filter. In some cases (e.g. crbug.com/993971),
that parent surface was very large, resulting in very large readbacks,
and poor performance. With this CL, which is a perf optimization, the
readback rect is intersected with the device viewport (expanded for
pixel moving filters) prior to readback. Pixels outside of the
viewport will not contribute to the visible filtered content.

Bug: 993971, 974789
Change-Id: I429c8dcb335b264873b3a3671225d19162ecfe27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761346
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689536}