[php-src] PR #24503: Add gd.png_ignore_warning INI setting

Pull Request: https://github.com/php/php-src/pull/24503 Author: cgsmith Closes GH-23674. Adds a gd.png_ignore_warning INI setting, following the design @devnexen suggested in the issue and mirroring gd.jpeg_ignore_warning. PNGs with an invalid iCCP chunk (e.g. Photoshop exports with a "known incorrect sRGB profile") currently emit an E_WARNING on every load, and the only way to hide it is @, which also hides real failures. - Off by default, so existing behaviour is unchanged. - When enabled, gdPngWarningHandler() drops recoverable libpng warnings. - gdPngErrorHandler() is untouched, so fatal errors are still reported. - Applies to imagecreatefrompng() and imagecreatefromstring(). - The flag is passed via new gdImageCreateFromPngEx()/gdImageCreateFromPngCtxEx() in the bundled libgd; the existing functions forward to them with the flag off. Open questions: - With an external libgd the setting is registered but has no effect. Happy to register it only for bundled builds instead. - Should the Ex functions also be proposed upstream to libgd? Tests: ext/gd/tests/gh23674.phpt with a small PNG fixture containing a malformed iCCP chunk. The full gd suite passes locally (macOS, bundled libgd, both non-ZTS and ZTS debug builds). Thanks to @adrian-schenk, who also offered to look into this on the issue.