Fix GH-23457: imagebmp() is extremely slow when writing to a file

[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() and imagegd2() go through the same context and were writing per byte too, so they get the same fix. imagexbm() goes through the same context but writes its output via putBuf rather than per-byte putC, so it was not affected by this bug and sees no change from this patch.

Fixes #23457

[devnexen]

is it a bug fix ? no, let s target master here. Thanks

[divinity76]

@devnexen using 40 seconds for something that should go in <0.1 seconds is a bug in my book

[devnexen]

ah ok I just read the bug report, ok that makes sense then.

[devnexen]

Might be best to rephrase the PR description a bit, e.g. imagexbm does not look fixed to me. As far as imagebmp goes however, it s looking good.

[lazerg]

You're right, imagexbm() writes through putBuf, not putC, so it never had this bug. Reworded the description to say that instead of grouping it with the others. NEWS and the fix itself only ever named imagebmp() so those didn't need a change.