Merge more of the image attachments codebase duplicated between Darwin and Windows.#1329

grynspan merged 4 commits intomainswiftlang/swift-testing:mainfrom

jgrynspan/merge-more-cgimage-iwicbitmapsource-codeswiftlang/swift-testing:jgrynspan/merge-more-cgimage-iwicbitmapsource-codeCopy head branch name to clipboard