SkPDF: Use type 2/3 shading for gradient shaders

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1925233003

Review-Url: https://codereview.chromium.org/1925233003
diff --git a/AUTHORS b/AUTHORS
index a6ade42..35cf14c 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -40,4 +40,5 @@
 Jose Mayol <jei.mayol@gmail.com>
 Linaro <*@linaro.org>
 Christian Plesner Hansen <plesner@t.undra.org>
-Marco Alesiani <marco.diiga@gmail.com>
\ No newline at end of file
+Marco Alesiani <marco.diiga@gmail.com>
+Adobe Systems Incorporated <*@adobe.com>