Use transparency when combining P frames from APNGs

[radarhere]

Helps #8440

When subsequent frames are loaded from an APNG image, transparency is considered for RGB core images, but not for P core images.

Pillow/src/PIL/PngImagePlugin.py

Lines 1061 to 1066 in 01bb78a

	if self.im.mode == "RGB" and "transparency" in self.info:
	mask = updated.convert_transparent(
	"RGBA", self.info["transparency"]
	)
	else:
	mask = updated.convert("RGBA")

This adds that, modelled after

Pillow/src/PIL/Image.py

Lines 1100 to 1103 in 01bb78a

	if isinstance(t, bytes):
	self.im.putpalettealphas(t)
	elif isinstance(t, int):
	self.im.putpalettealpha(t, 0)