https://en.cppreference.com/w/cpp/container/span/subspan
The real std::span::subspan in C++20 allows the "count" argument to
default to `std::dynamic_extent` (a fancy way of spelling ~0U). I didn't
think it would be worth adding `skstd::dynamic_extent`, but I did have
a use for an unbounded subspan, so I added a single-argument version to
SkSpan.
Change-Id: I297cc452cf2db727a3f9869ff8f46f3527e19370
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/529603
Reviewed-by: Arman Uguray <armansito@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
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