Corrected duration when combining multiple GIF frames into single frame

[radarhere]

Here is one of our tests, originally added in #4003

Pillow/Tests/test_file_gif.py

Lines 858 to 875 in 1c2f2c7

	@pytest.mark.parametrize(
	"duration", ([1000, 1500, 2000, 4000], (1000, 1500, 2000, 4000), 8500)
	)
	def test_identical_frames_to_single_frame(duration, tmp_path):
	out = str(tmp_path / "temp.gif")
	im_list = [
	Image.new("L", (100, 100), "#000"),
	Image.new("L", (100, 100), "#000"),
	Image.new("L", (100, 100), "#000"),
	]
	im_list[0].save(out, save_all=True, append_images=im_list[1:], duration=duration)
	with Image.open(out) as reread:
	# Assert that all frames were combined
	assert reread.n_frames == 1
	# Assert that the new duration is the total of the identical frames
	assert reread.info["duration"] == 8500

Saving multiple identical GIF frames results in a single frame. That PR changed GifImagePlugin to also give that single frame the combined duration. However, if you look at the test, it is saving three frames, but adding together a sequence of four durations. This PR ignores the fourth duration, so that a merged GIF is not longer than it would have been if the frames were not identical.

A second effect of this change is that when saving multiple identical GIF frames with an integer duration (see the last parameter in the test), rather than a sequence, the duration is multiplied by the number of frames, rather than just being the original integer. This again means that a merged GIF has the same total duration that it would have had if the frames were not identical.