PDF backend: don't apply paint alpha twice when we have a shader

Uncovers related bug in pdfium:
https://bugs.chromium.org/p/pdfium/issues/detail?id=1693

Bug: skia:11863
Change-Id: I764ccbfb5464f0288c66086826a1fd8bcf2ff8f7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/418544
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed