[qodelicious]

Changes proposed in this pull request:

• Document the compress_type save option for PNG. It gets passed to zlib as the deflate strategy, but it wasn't in the handbook (it came up in Saving PNG images with PIL is 4 times slower than saving them with OpenCV #5986 as a way to speed up saving)
• Lists the zlib strategy constants it takes and says the default is zlib.Z_FILTERED, which is what ZipEncode.c uses for PNG when it isn't set

I saved a gradient with each strategy to check they all work and that leaving it out gives the same file as Z_FILTERED. sphinx-lint is happy.

[radarhere]

the default is zlib.Z_FILTERED, which is what ZipEncode.c uses for PNG when it isn't set

This isn't quite correct. If the mode is P, it is Z_DEFAULT_STRATEGY.

Pillow/src/PIL/PngImagePlugin.py

Line 1140 in 0c8cc31

"P": ("P", b"\x08", b"\x03"),

Pillow/src/encode.c

Lines 655 to 658 in 0c8cc31

	if (rawmode == IMAGING_RAWMODE_P || rawmode == IMAGING_RAWMODE_PA) {
	/* disable filtering */
	((ZIPSTATE *)encoder->state.context)->mode = ZIP_PNG_PALETTE;
	}

Pillow/src/libImaging/ZipEncode.c

Lines 85 to 86 in 0c8cc31

	compress_type =
	(context->mode == ZIP_PNG) ? Z_FILTERED : Z_DEFAULT_STRATEGY;