Add GDI + Skia expectation for transparent.pdf

With GDI + Skia, transparent.pdf renders the same as with Skia. Add a
new copy of transparent_expected_skia.pdf.0.png so the corpus test
matches this file.

Update the presubmit to allow for 2 new renderer variations in the file
names:

1) gdi_agg
2) gdi_skia

Bug: pdfium:2056,pdfium:2080
Change-Id: I03ee6568e508fe254941f2db197e7de884a0e489
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/112110
Reviewed-by: Nigi <nigi@chromium.org>