Update corpus tests for late premultiplication

Updates corpus tests due to minor rounding differences caused by late
premultiplication.

Bug: pdfium:2011
Change-Id: Ie83d93c0090ebfea6b458f9854c6188b7d06546a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/108650
Reviewed-by: Nigi <nigi@chromium.org>