[Rounded Display] Add pixel tests for Rounded Display feature

These pixel tests verify that rounded display mask textures are
correctly drawn on a display.

Bug: b/331996916
Change-Id: I3a69c23897e4e1b75ad6b5cbb3f51b640bde67e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5408692


Commit-Queue: Zoraiz Naeem <zoraiznaeem@chromium.org>
Reviewed-by: Addison Luh <aluh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282753}