fix: use :background_color key in embed adjust_transparency

[hlindset]

Fixed a KeyError in adjust_transparency/3. It tried to read options.background instead of options.background_color. The bug only triggered on Image.embed/4 with extend_mode: :background on images with an alpha band, and 1-band images.

P.S. It looks like there's a dead branch in there, because Pixel.to_pixel always pads background_color to the image's full band count before adjust_transparency sees it, so length == bands is always true. Not fixed in this PR.

image/lib/image/options/embed.ex

Lines 176 to 184 in 5bb98af

	if length(options.background) == bands do
	options
	else
	options
	|> Map.put(
	:background_color,
	List.insert_at(options.background, -1, options.background_transparency)
	)
	end

(code from current main, so the key isn't fixed in the snippet)

Would it make sense to merge background_color and background_transparency into a single Image.Pixel-backed background option, so the alpha rides on the color and the reconcile logic goes away? It should be possible to keep the color/transparency pair as a deprecated alternative that folds into the single key.