Affine: keep libvips' native background instead of forcing :black to get alpha-aware fill

[hlindset]

• Image.affine/3, Image.translate/4 and Image.shear/4 no longer inject a default :background of :black. When :background is omitted, libvips keeps its own native fill, which is transparent on images with an alpha band, opaque black otherwise.
• Updated the :background option text to document the native default ("transparent for images with an alpha band, black otherwise") in place of "The default is :black."
• Added a ## Partially-transparent backgrounds section like in Expose :interpolate and route :background through Image.Pixel.to_pixel/2 in Image.rotate/3 #204.

I think this is more expected/ergonomic? An oversight on my part in #202.