SkPDF: clean up image shader code

simplify dedup key

Change-Id: I0fd2a7357130c73dbfa799ade0ff443b74d72993
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218542
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
2 files changed