Split APNG frame durations too large for a single fcTL chunk

[niukanen1]

Fixes #10063. Alternative to #10070

Saving an APNG with a frame duration whose fractional delay has a numerator over the 16-bit fcTL limit raised ValueError: cannot write duration instead of saving, even when the individual frame delays were representable — the merge of identical consecutive frames can push a summed duration over the limit (the example from the issue: two identical frames with duration=[32768, 32769] merge into a single 65537 ms frame that no longer fits).

Following the direction from #10063 (comment), the duration is now split across consecutive identical frames, each with a delay that fits, so the animation replays the full duration. The split only triggers when a delay doesn't fit, and the total duration is preserved exactly.

from PIL import Image

frame_a = Image.new("RGB", (1, 1), "green")
frame_b = Image.new("RGB", (1, 1), "red")

# used to raise ValueError: cannot write duration
frame_a.save("output.png", format="PNG", save_all=True,
             append_images=[frame_b, frame_b], duration=[100, 32768, 32769])

[radarhere]

I've created #10070 as an alternative that I think is simpler. See what you think.

[niukanen1]

Agreed, yours is simpler and it also keeps the animation where the split approach loses it: two identical frames with something like [32768000, 32768000] combine into a single frame and the existing one-frame short-circuit writes a static PNG, while #10070 keeps both frames. Closing in favour of #10070