Unify background and interpolation handling for mapim transformations

[hlindset]

mapim-based transformations now handle transparent and semi-transparent backgrounds consistently through shared option validation and conditional alpha premultiplication.

• ripple: added :background and :interpolate options
• to_polar_coordinates: added :background and :interpolate options
• to_rectangular_coordinates: added :interpolate option, and hard-coded extend: :VIPS_EXTEND_COPY. Boundary samples will now be clamped to source edge color instead of blending towards the zero-fill
• distort: added :background, :interpolate (default is still :bicubic as before), and :extend_mode options (:background and :copy accepted)
• warp_perspective: now preserves image alpha and reproduces non-opaque backgrounds correctly, added :interpolate option
• straighten_perspective: now preserves image alpha and reproduces non-opaque backgrounds correctly, added :interpolate option
• map: now reproduces non-opaque backgrounds correctly, added :interpolate option

The :interpolate option defaults to :bilinear, except for distort, which retains its existing :bicubic default.

For transformations that accept :background, the default (when omitted or nil) is libvips' all-zero fill: black for images without alpha, transparent for images with alpha.

The existing premultiplication handling was extracted into a shared helper, and all affected operations are now routed through a shared mapim helper which applies explicit premultiplication when required.

I also removed the following doc note from to_rectangular_coordinates because the roundtrip resamples the image twice, so some accuracy loss is expected, and forcing :extend to :VIPS_EXTEND_COPY also removes the background-bleed artifacts.

  ## Notes
  Roundtrip to polar and back to rectangular
  coordinates displays some image distortion,
  likely due to rounding errors in float
  arithmetic. Further study is required.

Breaking changes:

• warp_perspective, straighten_perspective now preserve alpha instead of always flattening, so output band count and pixels may change.
• map now defaults to transparent black (all-zero) for exposed pixels in images with alpha (previously opaque black)
• to_rectangular_coordinates now clamps boundary samples to the source edge (extend: :VIPS_EXTEND_COPY)
• Image.Options.WarpPerspective was removed and replaced by Image.Options.Mapim, so direct references to the old module no longer compile.

[kipcole9]

Wow, such amazing work, thank you very much! I'm travelling at the moment but will review and aim to merge by the end of the weekend.