Use actual widths for srcset descriptors of undersized images

[jan-23]

Fixes #53

Problem

When a source image is narrower than a srcset entry's w descriptor (and upscaling is disabled), the generated derivative ends up narrower than declared — but the srcset still advertises the declared width. For a 1500px image with 1000w/2000w/3000w entries, the browser is told it can pick a 3000px image, while the file is actually 1500px.

Change

After processing each srcset entry, the actual width of the derivative is compared with its w descriptor (new helper get_srcset_descriptor()):

• If the image is at least as wide as declared, the descriptor is kept as-is.
• If it is narrower, the descriptor is replaced by the real width (e.g. 1500w instead of 2000w).
• If that real width is already present in the srcset, the entry is omitted.

For the example from the issue the result is:

srcset="/large/1000w/img_b.jpg 1000w, /large/2000w/img_b.jpg 1500w"

This applies to responsive-image and both picture code paths. Density (x) descriptors are untouched, and if an image cannot be processed the declared descriptor is kept.

Differences from the proposal in the issue

• Derivative files are still generated. The width of a derivative is only known after running its operations (which may crop, upscale, or be arbitrary callables), and the existing cache relies on the output file existing. Not writing duplicates would mean re-processing them on every build, or adding persistent state. Because nothing links to these files, I kept this PR minimal; happy to add pruning in a follow-up if you think it's worthwhile.
• The original image is not added as a bare fallback. A srcset cannot mix a candidate without a descriptor with w candidates. The derivative listed with its real width serves the same purpose while preserving format conversion and other operations.

Tests

• Unit tests for get_srcset_descriptor()
• An end-to-end test with a real image (pelican-bird.jpg, 1024px wide) and scale_out … False entries up to 3000w
• A <picture> test with a mocked undersized source
• Existing HTML-generation tests now use a mock that returns each derivative's declared width, so their expectations are unchanged

README and RELEASE.md updated.

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