Implement preserveSvg flag for srcSet()

[mgax]

Fixes #369

[zerolab]

This all LGTM.

Left a provocation (well, a challenge question) and would love to hear your thought as an implementer

[zerolab]

Based on a conversation in the Wagtail Slack, I am very much inclined to make preserve_svg default to True here (and on the rendition)

ie. until Wagtail (well, Willlow) can actually rasterize SVGs, the only way to work with SVGs is to use preserve_svg in Grapple, and preserve-svg in regular templates, which adds so much extra effort.

What d'ya think?

[mgax]

Yes, I think it very much makes sense. I can't imagine a scenario where you would not want preserveSvg: true as a default, even if it were possible to rasterize them.

We need to make sure this doesn't break on Wagtail < 5 though; I remember the code was relying on the fact that, on old Wagtails, it would not be possible to change the default False value.

[zerolab]

I think it should be fine. as preserve_svg filters out non-SVG filters -

wagtail-grapple/grapple/types/images.py

Lines 153 to 161 in d8bb922

	if instance.is_svg() and preserve_svg:
	# when dealing with SVGs, we want to limit the filter specs to those that are safe
	filter_specs = to_svg_safe_spec(filter_specs)
	if not filter_specs:
	raise TypeError(
	"No valid filter specs for SVG. "
	"See https://docs.wagtail.org/en/stable/topics/images.html#svg-images for details."
	)

[mgax]

Yes, but to_svg_safe_spec is imported only for Wagtail >= 5; just wanted to mention this, as there don't seem to be a CI matrix that includes older Wagtails:

wagtail-grapple/grapple/types/images.py

Lines 24 to 25 in d8bb922

	if WAGTAIL_VERSION > (5, 0):
	from wagtail.images.utils import to_svg_safe_spec

---

I can implement the change; do you feel it should be part of this PR or separate?

[zerolab]

we run tox with tox-gh-actions in CI, so the matrix is in https://github.com/torchbox/wagtail-grapple/blob/main/tox.ini

let's follow up in a separate PR. We'd want to wrap if instance.is_svg() and preserve_svg: in an if WAGTAIL_VERSION > (5, 0) to be sure

[mgax]

we run tox with tox-gh-actions in CI, so the matrix is in https://github.com/torchbox/wagtail-grapple/blob/main/tox.ini

@zerolab I think something is missing. I couldn't find a CI job that mentioned an old version of Wagtail so I made a branch to see if I can break it. AFAICT this should have failed CI: mgax@7b52a38