Adopt std::span in GIFImageReader.
Pull request: https://github.com/WebKit/WebKit/pull/27229
Committed 277476@main (6c6ce2d397a5): <https://commits.webkit.org/277476@main> Reviewed commits have been landed. Closing PR #27229 and removing active labels.
<rdar://problem/126450538>
Regression: bug #274027