Do not use first frame duration for other frames when saving APNG images

[radarhere]

Resolves #8102

When saving APNG images, if there is no duration argument given to the save() command, then duration is pulled from the first image's info dictionary.

Pillow/src/PIL/PngImagePlugin.py

Lines 1107 to 1108 in 1794a94

	def _write_multiple_frames(im, fp, chunk, rawmode, default_image, append_images):
	duration = im.encoderinfo.get("duration", im.info.get("duration", 0))

That value is later re-used for subsequent frames, leading to the unexpected behaviour in the issue.

Pillow/src/PIL/PngImagePlugin.py

Lines 1170 to 1171 in 1794a94

	if "duration" not in encoderinfo:
	encoderinfo["duration"] = duration

This PR corrects this, using GifImagePlugin's _write_multiple_frames() as a model.

[hugovk]

This has a conflict.