Improve SgiImagePlugin test coverage

[radarhere]

Looking at SgiImagePlugin, I see that an error for the incorrect number of bands will never be raised.

Pillow/src/PIL/SgiImagePlugin.py

Lines 160 to 176 in 507fefb

	dim = 3
	# X Dimension = width / Y Dimension = height
	x, y = im.size
	if im.mode == "L" and y == 1:
	dim = 1
	elif im.mode == "L":
	dim = 2
	# Z Dimension: Number of channels
	z = len(im.mode)
	if dim in {1, 2}:
	z = 1
	# assert we've got the right number of bands.
	if len(im.getbands()) != z:
	msg = f"incorrect number of bands in SGI write: {z} vs {len(im.getbands())}"
	raise ValueError(msg)

Only L, RGB and RGBA modes are supported when saving.

Pillow/src/PIL/SgiImagePlugin.py

Lines 137 to 139 in 507fefb

	def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
	if im.mode not in {"RGB", "RGBA", "L"}:
	msg = "Unsupported SGI image mode"

In the first code snippet, you can see that z = len(im.mode) would be set z to the correct values of 1 for L, 3 for RGB and 4 for RGBA. It is later set to 1 if dim is 1 or 2, but dim only has those values for L mode images, which should be 1 anyway.

So the error can be removed.

After some code simplification, this adds coverage for the lines missing at https://app.codecov.io/gh/python-pillow/Pillow/blob/main/src%2FPIL%2FSgiImagePlugin.py

[hugovk]

Shall we use more a descriptive name? (and below)

Suggested change

	dim = 1 if y == 1 else 2
	else:
	dim = 3
	dimensions = 1 if y == 1 else 2
	else:
	dimensions = 3

[radarhere]

I've pushed a commit with dimension. It matches

Pillow/src/PIL/SgiImagePlugin.py

Lines 73 to 74 in 1788ab7

	# dimension : 1, 2 or 3 (depending on xsize, ysize and zsize)
	dimension = i16(s, 4)

and the NAME in 'The Header' at https://paulbourke.net/dataformats/sgirgb/sgiversion.html