Fixed seeking to an L frame in a GIF

[radarhere]

Resolves #6571

When pasting an L frame onto an RGB(A) GIF, convert the frame to RGB(A) first.

The test image was created using a modified Pillow, so that there is no global palette, and the image is comprised of a frame with a palette, and then two frames without. This means that the first frame is P mode, the second is L mode, combining to make RGB, and then there is an L mode frame pasted onto that RGB frame.

[radarhere]

I've added a commit to address #6610 (comment)

That comment found a GIF where on a later frame, the palette is "not needed" because the frame is grayscale. The problem is that Pillow then concludes that since there isn't a palette, it should fallback to the global palette.

Pillow/src/PIL/GifImagePlugin.py

Lines 275 to 301 in 243402e

	if self._is_palette_needed(p):
	palette = ImagePalette.raw("RGB", p)
	# image data
	bits = self.fp.read(1)[0]
	self.__offset = self.fp.tell()
	break
	else:
	pass
	# raise OSError, "illegal GIF tag `%x`" % s[0]
	s = None
	if interlace is None:
	# self._fp = None
	raise EOFError
	self.__frame = frame
	if not update_image:
	return
	self.tile = []
	if self.dispose:
	self.im.paste(self.dispose, self.dispose_extent)
	self._frame_palette = palette or self.global_palette

This is incorrect, as the frame is specifying custom palette behaviour, it's just that custom behaviour is to have a grayscale palette.