This is a simple but everyday use case in image libraries. Currently,
there is one example in this library and it is lengthy and involved.

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Change-Id: Idca527d97c095af88755446e1548fa2b8ace7eb0
GitHub-Last-Rev: f5743c8ef337a8b7a5126db21d784593f4f53c8a
GitHub-Pull-Request: golang/go#25616
Reviewed-on: https://go-review.googlesource.com/114939
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>