Update Skia expectations in fx/new/shading_2

CL [1] changed the some Skia layers from type `kOpaque_SkAlphaType` to
`kPremul_SkAlphaType` which affected the edges of images. This CL
updates the affected corpus tests in fx/new/shading_2.

[1] https://pdfium-review.googlesource.com/100930

Bug: pdfium:1855
Change-Id: Ib7aae675e20930f2545ea46949699177e5310ee7
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/102391
Reviewed-by: K. Moon <kmoon@chromium.org>