Update corpus expectations for axial shading incremental stepping

Updates AGG expectations for 4_2, shading, shading1, and shading_2_8
for the affine stepping optimization in
https://pdfium-review.googlesource.com/c/pdfium/+/161830.

Bug: 42270903
Change-Id: I8a4306132ccdd81c5c4197c1d6817b3bd120c119
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/161850
Reviewed-by: Lei Zhang <thestig@chromium.org>