Allow SkSpan to be created from Container&&

While this is a bit dangerous for the user, it does allow for general
use of expression local containers. Also, std::span allows it. Doing
this means that the initializer_list deduction guide can be removed.

Change-Id: Ifa5d1bcc36a78cc259231b68ee258d96bfe06370
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/709206
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed