Add subspan() to SkZip

Change-Id: I54916c7e14646b8a66dd8341a1cb4b46ba8dba2c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254737
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Auto-Submit: Herb Derby <herb@google.com>
2 files changed