Ensure that pointers/arrays are valid until flush in threaded backend

Bug: skia:7672 skia:7414
Change-Id: Ia000781401fe7b1df8040abb97692e6ca35b312f
Reviewed-on: https://skia-review.googlesource.com/118626
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
2 files changed