Add WebP shrink-on-load support via scale parameter

[JakeChampion]

Expose the libvips webpload "scale" parameter as WebpScaleFactor on ImportParams, allowing callers to downscale WebP images during decode

Note

Add WebP shrink-on-load via ImportParams.WebpScaleFactor and pass LoadParams.webpScale to the WebP loader scale option

Introduce a WebP scale-in parameter, thread it through createImportParams to LoadParams.webpScale, set it in set_webpload_options, and add tests for dimension changes and option string output.

📍Where to Start

Start with set_webpload_options in foreign.c, then review createImportParams in foreign.go and the ImportParams additions in image.go.

^{Macroscope summarized dd8f50b.}