Do not modify previous frame when calculating delta in PNG

[radarhere]

Helps the original code in #6672

When iterating through frames to save a PNG, the following code

Pillow/src/PIL/PngImagePlugin.py

Lines 1130 to 1138 in c924fd8

	if prev_disposal == Disposal.OP_BACKGROUND:
	base_im = previous["im"]
	dispose = Image.core.fill("RGBA", im.size, (0, 0, 0, 0))
	bbox = previous["bbox"]
	if bbox:
	dispose = dispose.crop(bbox)
	else:
	bbox = (0, 0) + im.size
	base_im.paste(dispose, bbox)

actually changing the previous frame with the paste operation.

This PR fixes that by making base_im a copy() instead.