Fix warp_perspective/straighten_perspective :extend_mode silent no-op and correct its docs

[hlindset]

Fixes the :extend_mode option on warp_perspective/straighten_perspective, which was silently a no-op.

Root cause: Image.Options.WarpPerspective validated :extend_mode and passed it straight through to Vix.Vips.Operation.mapim/3, but the operation's option is named :extend. mapim silently ignores unknown options, so the value never took effect, and output was always rendered with mapim's own default (extend: background).

• Image.Options.WarpPerspective now renames :extend_mode to :extend during validation, so the value reaches mapim/3.
• Default :extend_mode changed from :black to :background. The configured :black was never actually in use. The actual rendered default was mapim's background. Setting the configured default to :background keeps output at the default identical to before.
• Corrected the :extend_mode docs on warp_perspective/4, warp_perspective!/4, straighten_perspective/3 and straighten_perspective!/3. Like affine, mapim's extend only colours the thin antialiased fringe at the resampled content edge, not the uncovered canvas (which is filled by :background).

Anyone who has explicitly set :extend_mode to a value other than :background will see changes to the fringes around the resampled content.

[kipcole9]

Wow, you're doing some amazing and truly appreciated work. Thank you. I will consolidate all your work and get a new release out by my Wednesday morning (UTC+10).