When saving multiple PNG frames, convert to mode rather than raw mode

[radarhere]

Resolves #8081

The issue points out that _write_multiple_frames converts images to rawmode

Pillow/src/PIL/PngImagePlugin.py

Line 1125 in bbe1eff

im_frame = im_frame.convert(rawmode)

where rawmode is

Pillow/src/PIL/PngImagePlugin.py

Line 1297 in bbe1eff

rawmode, bit_depth, color_type = _OUTMODES[mode]

Pillow/src/PIL/PngImagePlugin.py

Lines 1052 to 1069 in bbe1eff

	_OUTMODES = {
	# supported PIL modes, and corresponding rawmode, bit depth and color type
	"1": ("1", b"\x01", b"\x00"),
	"L;1": ("L;1", b"\x01", b"\x00"),
	"L;2": ("L;2", b"\x02", b"\x00"),
	"L;4": ("L;4", b"\x04", b"\x00"),
	"L": ("L", b"\x08", b"\x00"),
	"LA": ("LA", b"\x08", b"\x04"),
	"I": ("I;16B", b"\x10", b"\x00"),
	"I;16": ("I;16B", b"\x10", b"\x00"),
	"I;16B": ("I;16B", b"\x10", b"\x00"),
	"P;1": ("P;1", b"\x01", b"\x03"),
	"P;2": ("P;2", b"\x02", b"\x03"),
	"P;4": ("P;4", b"\x04", b"\x03"),
	"P": ("P", b"\x08", b"\x03"),
	"RGB": ("RGB", b"\x08", b"\x02"),
	"RGBA": ("RGBA", b"\x08", b"\x06"),
	}

and so may not always be a Pillow image mode.