Consider default image when selecting mode for PNG save_all

[radarhere]

Resolves #7459

Sequel to #6610

1. While that PR theoretically considers all images, and "upgrades" images to more complex modes, it did not consider the default image.
   

   Pillow/src/PIL/PngImagePlugin.py

   Lines 1230 to 1239 in aaa7587

   	if default_image:
   	chain = itertools.chain(append_images)
   	else:
   	chain = itertools.chain([im], append_images)
   	for im_seq in chain:
   	for im_frame in ImageSequence.Iterator(im_seq):
   	modes.add(im_frame.mode)
   	for mode in ("RGBA", "RGB", "P"):
   	if mode in modes:
   	break

   
   This means that when the default image is saved with the chosen mode, it may not be the ideal choice. This PR considers the mode of the default image, and converts the default image before saving if needed.

2. Because that PR made it so that P images are "upgraded" to RGB if there are any RGB images present, a RGB to P conversion will no longer occur at
   

   Pillow/src/PIL/PngImagePlugin.py

   Lines 1102 to 1110 in aaa7587

   	for im_seq in chain:
   	for im_frame in ImageSequence.Iterator(im_seq):
   	if im_frame.mode == rawmode:
   	im_frame = im_frame.copy()
   	else:
   	if rawmode == "P":
   	im_frame = im_frame.convert(rawmode, palette=im.palette)
   	else:
   	im_frame = im_frame.convert(rawmode)

   
   meaning that the palette argument is unneeded.
   

   Pillow/src/PIL/Image.py

   Lines 907 to 909 in aaa7587

   	:param palette: Palette to use when converting from mode "RGB"
   	to "P". Available palettes are :data:`Palette.WEB` or
   	:data:`Palette.ADAPTIVE`.