Expose :interpolate and route :background through Image.Pixel.to_pixel/2 in Image.rotate/3

[hlindset]

• Image.rotate/3 now exposes :interpolate (:nearest, :bilinear (default), :bicubic, :lbb, :nohalo, :vsqbs), resolved to a Vix.Vips.Interpolate via Image.Options.Rotate. It has no effect on rotations by a multiple of 90, which copy source pixels directly.
• :background now accepts all valid Image.Pixel.to_pixel/2 values, or :average, matching Image.affine/3. Previously it only accepted a bare numeric list and could not express RGBA. Default is kept as libvips' native default (transparent on alpha images, opaque black otherwise).
• Image.Options.Rotate.invalid_option/1 now returns an %Image.Error{} - is this ok to just switch, or would you rather I kept that out?

Docs:

• Added a ## Partially-transparent backgrounds section. A fully opaque or fully transparent :background is reproduced exactly, while a partially transparent one is scaled by max / alpha because libvips injects the fill into premultiplied-alpha space and unpremultiplies on output. For an exact partial-alpha fill, the section documents a rotate over transparent then Image.compose/2 recipe.
• Replaced the vague/inaccurate displacement notes with a ## Displacements section reusing the affine wording for :idx/:idy/:odx/:ody.

A PR that changes Image.affine/3 to use libvips native default (probably the expected behaviour) instead of explicit :black as the default (+ a note about the alpha background caveat) is coming up. Missed these nuances in the original affine PR.