[INTEROP 2025] Add fuzzy diff to backdrop-filter-basic-opacity-2

This CL adds fuzziness to backdrop-filter-basic-opacity-2, to account
for subtle differences in rendering that can occur in different
instantiations of the SKIA renderer. Specifically, when blending
a color value of 128 with its inverse due to a filter of invert(),
the result may be either 128 or 127. Visually, these results are
almost identical, and well within tolerance.

Extensive investigation has been done into this behavior, and the
behavior of the backdrop filter-related render code in both the
passing and failing case is identical, and the failure is likely
caused by rendering differences closer to the GPU. This should not
be considered a failing test.

Bug: 40115360, 40181625
Change-Id: I05c70615d6ad8cfef8ebcfd372709170e19298ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6916525
Commit-Queue: Claire Chambers <clchambers@microsoft.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Olga Gerchikov <gerchiko@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1511163}