Only compare to previous frame when checking for duplicate GIF frames while saving

[radarhere]

Resolves #6785

When saving GIFs, a disposal value of 2 means that after the frame, we should "Restore to background color".

For that scenario, GifImagePlugin correctly compares the new frame to the background color, so that the new frame is optimised by only including the parts that differ from the background color.

Pillow/src/PIL/GifImagePlugin.py

Lines 601 to 618 in 4c9ed12

	if encoderinfo.get("disposal") == 2:
	if background_im is None:
	color = im.encoderinfo.get(
	"transparency", im.info.get("transparency", (0, 0, 0))
	)
	background = _get_background(im_frame, color)
	background_im = Image.new("P", im_frame.size, background)
	background_im.putpalette(im_frames[0]["im"].palette)
	base_im = background_im
	else:
	base_im = previous["im"]
	if _get_palette_bytes(im_frame) == _get_palette_bytes(base_im):
	delta = ImageChops.subtract_modulo(im_frame, base_im)
	else:
	delta = ImageChops.subtract_modulo(
	im_frame.convert("RGB"), base_im.convert("RGB")
	)
	bbox = delta.getbbox()

However, GifImagePlugin then decides that if there is no difference, the frame is identical to the previous frame.

Pillow/src/PIL/GifImagePlugin.py

Lines 619 to 620 in 4c9ed12

	if not bbox:
	# This frame is identical to the previous frame

That is not true when disposal is 2. It would be true if disposal weren't 2, and we were comparing this frame to the previous frame, but we are instead comparing it to the background color.

So this PR instead only compares the previous frame with this frame when checking for duplicate frames.