Add wpt testing for backdrop filter mirror edge mode.
This adds a wpt test covering the new mirror edge mode expectation in
https://drafts.fxtf.org/filter-effects-2/#backdrop-filter-operation
Bug: 40040614
Change-Id: I278beefa6f0568e09b00512888adbff42542bd2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631928
Reviewed-by: Mason Freed masonf@chromium.org
Commit-Queue: Robert Flack flackr@chromium.org
Cr-Commit-Position: refs/heads/main@{#1316077}