[php-src] PR #23460: Fix GH-23457: imagebmp() is extremely slow when writing to a file

Pull Request: https://github.com/php/php-src/pull/23460 Author: lazerg imagebmp() writes its pixel data a byte at a time, and the gd stream context turned each of those bytes into its own php_stream_write() call. PHP streams do no write buffering, so a 1920x1080 truecolor image cost about six million write syscalls. libgd's own FILE context does not show this because stdio buffers for it. Buffering the stream context in 8 KB chunks takes that image from 9.5s to 0.02s here, with byte-identical output. imagewbmp(), imagegd(), imagegd2() and imagexbm() go through the same context and were writing per byte too. Fixes #23457