Add gradient_many_stops.pdf

Add a test file that renders correctly with Skia, but not with AGG. The
test expectation is the Skia output.

Bug: pdfium:489
Change-Id: Ibbaccacc8e4d8d47929a5c4c6be7a24f03e1e91f
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/104770
Reviewed-by: K. Moon <kmoon@chromium.org>