Do not update palette for L mode GIF frame

[radarhere]

Resolves #8921

When a GIF frame doesn't have a global palette, and an individual frame has a palette where the colours sequentially move from (0, 0, 0) to (255, 255, 255), or even if the palette is less than 256 colours in length and still follows that progression, Pillow reads that as an L mode frame.

Pillow/src/PIL/GifImagePlugin.py

Lines 93 to 97 in 58e4874

	def _is_palette_needed(self, p: bytes) -> bool:
	for i in range(0, len(p), 3):
	if not (i // 3 == p[i] == p[i + 1] == p[i + 2]):
	return True
	return False

Pillow/src/PIL/GifImagePlugin.py

Lines 291 to 294 in 58e4874

	if self._is_palette_needed(p):
	palette = ImagePalette.raw("RGB", p)
	else:
	palette = False

Pillow/src/PIL/GifImagePlugin.py

Line 315 in 58e4874

self._frame_palette = palette if palette is not None else self.global_palette

Pillow/src/PIL/GifImagePlugin.py

Line 430 in 58e4874

temp_mode = "P" if self._frame_palette else "L"

GIF frames may have transparency though, and

Pillow/src/PIL/GifImagePlugin.py

Lines 479 to 480 in 58e4874

	if self._frame_transparency is not None:
	self.im.putpalettealpha(self._frame_transparency, 0)

is not able to accomodate an L mode frame with transparency.

This fixes that.