Make srcset URL building overridable in ImageScaling

[jensens]

Summary

The srcset() method in ImageScaling hardcodes scale URL construction inline:

f'{self.context.absolute_url()}/@@images/{scale["uid"]}.{extension} {scale["width"]}w'

This makes it impossible for custom scaling backends (e.g. external image services like Thumbor, Imgix, Cloudinary) to generate direct URLs in srcset without copying the entire srcset() method — which is fragile and hard to keep in sync with upstream changes (like #197).

Proposed change

Extract the URL-building into an overridable method:

class ImageScaling:

    def _scale_url(self, scale, fieldname):
        """Build the URL for a single scale entry.

        Override this in subclasses to generate URLs for external
        image services instead of @@images/{uid} paths.
        """
        extension = scale["mimetype"].split("/")[-1].lower()
        return f'{self.context.absolute_url()}/@@images/{scale["uid"]}.{extension}'

    def srcset(self, fieldname, ...):
        # ... existing logic ...
        for width, height in self.available_sizes.values():
            if width <= original_width:
                scale = storage.pre_scale(...)
                url = self._scale_url(scale, fieldname)
                srcset_urls.append(f'{url} {scale["width"]}w')

This would allow custom backends to override just _scale_url():

class ThumborImageScaling(ImageScaling):

    def _scale_url(self, scale, fieldname):
        # Generate a direct Thumbor URL instead of @@images/{uid}
        blob_ids = get_blob_ids(getattr(self.context, fieldname))
        if blob_ids:
            return thumbor_url(
                server_url=cfg.server_url,
                zoid=blob_ids[0], tid=blob_ids[1],
                width=scale["width"], height=scale["height"],
                ...
            )
        return super()._scale_url(scale, fieldname)

Motivation

We're building plone.pgthumbor, which replaces ZODB-stored image scales with Thumbor URL redirects. Currently @@images/{uid} URLs in srcset cause a 302 redirect per loaded image. While functional, direct URLs in srcset would eliminate the redirect hop entirely.

The same hook would benefit any external image processing integration.