Update corpus expectations due to Skia transparency fix

Update Skia rendering expectations so they pass when
https://pdfium-review.googlesource.com/112352 has been applied. Also
add an expectation file for bug_668762.pdf so it passes with GDI + AGG.

Bug: pdfium:2081
Change-Id: I55642ac0848537a7fd61da8140b9ccbfede2ba62
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/112332
Reviewed-by: Nigi <nigi@chromium.org>
8 files changed