Only combine PNG frames if the combined duration is still writable

[radarhere]

Resolves #10063

If PNG's fcTL chunk cannot express the delay numerator in 2 bytes, we raise an error.

Pillow/src/PIL/PngImagePlugin.py

Lines 1302 to 1305 in e112f53

	delay = Fraction(frame_duration / 1000).limit_denominator(65535)
	if delay.numerator > 65535:
	msg = "cannot write duration"
	raise ValueError(msg)

If consecutive PNG frames are identical, we combine them to result in a smaller file.

Pillow/src/PIL/PngImagePlugin.py

Lines 1257 to 1264 in e112f53

	bbox = delta.getbbox(alpha_only=False)
	if (
	not bbox
	and prev_disposal == encoderinfo.get("disposal")
	and prev_blend == encoderinfo.get("blend")
	and "duration" in encoderinfo
	):
	previous.encoderinfo["duration"] += encoderinfo["duration"]

Let's not have these two features work against each other. If the combined duration of frames cannot be expressed in an fcTL chunk, then let's not combine them.