Image.write/3: accept :average or Pixel.t() as :background

[hlindset]

This PR makes it possible to set Image.write/3's :background option to :average or Pixel.t().

The original :background handling for Image.write/3 just guarded is_pixel, and then pushed the raw value into vix, causing protocol Enumerable not implemented for ... for anything other than numeric lists.

image/lib/image/options/write.ex

Lines 301 to 303 in 5bb98af

	defp validate_option({:background, background}, options, _image_type) when is_pixel(background) do
	{:cont, options}
	end

Open questions:

• Adding :average required threading the image through all validation functions, so all function heads had to be updated. If you'd prefer I could do it in a resolve/validate step after the main validation has completed.
• I added Image.Options.Write.invalid_option/1 for errors not bound to a specific image type, but the return type of that is %Image.Error{}, not binary() like Image.Options.Write.invalid_option/2. Is that ok, or should I switch both to either %Image.Error{} or binary()?