Add a test case for gradients with opacity.

BUG=chromium:668762

Change-Id: I99de143c6c981336994c17d8470dd9c1862bdfa3
Reviewed-on: https://pdfium-review.googlesource.com/39290
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
2 files changed