Allow cloning from another stack with an offset-from-top.

This requires an extra immediate in the Builder instruction, so
I initially left it out, but it will be useful when doing more
complex ops (e.g. matrix multiplication).

Change-Id: I9ec63ac89d804bcda24ca1c5888bf8394fc9ac2a
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/628916
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
4 files changed