Unify the opacity vocabulary + fix alpha scaling for 16-bit and scRGB images

[hlindset]

This work unifies the naming for setting and operating on opacities. Previously transparency, alpha and opacity were all used. The naming is now split in two: opacity is the value the caller sets, unscaled and independent of the image's interpretation, e.g. Image.add_alpha(image, opacity), background: {:red, opacity: 0.5}. alpha is kept for operations and values that touch the alpha band directly. On the way there, I also fixed three bugs, and tightened the error contract of Image.Pixel.

All functions that take an opacity now accept the t:Image.Pixel.opacity/0 type: 8-bit notation 0..255, float notation 0.0..1.0, :transparent, and :opaque.

Bugs fixed

• Breaking: Image.add_alpha/2 now correctly scales the alpha band it adds to the image's interpretation. Previously wrote an 8-bit band into float and 16-bit alpha bands. Interpretations with 8-bit alpha bands are unaffected, including Lab and LCH.
• Breaking: Image.Pixel.to_pixel/3 no longer quantizes a float :opacity to a byte before scaling it. Changes alpha values for 16-bit and scRGB images given a float opacity that is not representable in the 8-bit range.
• Image.Pixel.to_pixel/3 now applies the :opacity option to a color given as a list of numbers.

Added

• Image.Pixel.put_alpha/3, Image.Pixel.put_alpha!/3: Returns a pixel with its alpha component set to a given opacity, scaled to the image's alpha band. Returns an unchanged pixel when no alpha band is present.
• Image.Pixel.alpha_for/2, Image.Pixel.alpha_for!/2: Scales an opacity to the alpha band of a given image, whose range depends on the interpretation.
• Image.Pixel.opacity_fraction/1, Image.Pixel.opacity_fraction!/1: Takes t:Image.Pixel.opacity/0 and returns a fraction of full opacity.

Changed

• The :opacity options of Image.Shape and the :background_fill_opacity / :background_stroke_opacity options of Image.Text.text/2 accept any t:Image.Pixel.opacity/0 rather than only a float.
• Breaking: The :opacity option of Image.drop_shadow/2 accepts any t:Image.Pixel.opacity/0 rather than any number in 0.0..1.0. opacity: 1 previously meant fully opaque and is now 8-bit notation for 1/255, as it is everywhere else. Use 1.0 instead. Only 1 changes meaning, since 0 remains fully transparent.
• Breaking: Image.Pixel returns an Image.Error on every error path. An unsupported interpretation was {:error, binary()} and is now reason: :unsupported_interpretation. Invalid colors were the Color library's own exceptions, and are now reason: :invalid_color with the color in value and the original text as the message. Reaches every function that resolves a color, including the :background options.
• Breaking: An invalid opacity reports reason: :invalid_opacity instead of :invalid_transparency.
• Image.add_alpha/2 now accepts the full t:Image.Pixel.opacity/0 type, whereas previously it only accepted 0..255, :transparent, and :opaque.
• Image.add_alpha/2 returns {:error, %Image.Error{reason: :invalid_opacity}} for an invalid opacity, where it previously raised FunctionClauseError.

Removed

• Breaking: Removes Image.Pixel.transparency/1, Image.Pixel.max_opacity/0 and Image.Pixel.min_opacity/0. Use Image.Pixel.alpha_for/2 for a value to write into an image's alpha band, or Image.Pixel.opacity_fraction/1 for the opacity itself.
• Breaking: Removes the t:Image.transparency/0 and t:Image.Pixel.transparency/0 types. Replaced by t:Image.Pixel.opacity/0.
• Breaking: Removes the :alpha option of Image.Pixel.to_pixel/3 and the {color, alpha: alpha} background form. Use :opacity and {color, opacity: opacity}, which are the same options under new names.
• Breaking: Removes :none as an opacity, though not as a color. Use :transparent, which it was an exact synonym for.

Open questions

• The only naming wrinkle left, where opacity means something else, is Image.opacity/2. One possibility is to rename it to e.g. Image.multiply_alpha/2?
• It's possible to make a variant of Image.Pixel.alpha_for/2 that accepts an interpretation atom, so alpha resolution can be done for an interpretation without having an image at hand. Not needed by any of the internal call sites, so I didn't add that. Easy to add, if you think it'll be useful?