ImageConverter: ignore '?' image URIs

[mgeier]

While #10073 has fixed my minimal example, it wasn't enough to fix my original problem spatialaudio/nbsphinx#559.

It turns out that on top of data: URIs, remote URIs have also been automatically handled as "not supported" and therefore unnecessarily invoked the image converters.

This PR ignores all '?' images, which includes data: URIs, therefore I've reverted the changes from #10099, which are no longer necessary.

I still don't know what '?' means exactly, it would probably be nice to add a comment explaining what that is and why it's ignored.

Example project for experiments: https://github.com/mgeier/rsvgconverter-bug

[tk0miya]

I consider the ImageConverter should handle both local and remote images. But, essentially, current implementation mainly supports local images only. So this proposal is a correct approach.

Note: One of my extension, sphinxcontrib-cacoo supports remote images by overriding match() method.
https://github.com/sphinx-contrib/cacoo

[tk0miya]

I still don't know what '?' means exactly, it would probably be nice to add a comment explaining what that is and why it's ignored.

'?' represents "nonlocal" images.

sphinx/sphinx/environment/collectors/asset.py

Lines 39 to 42 in 1e1f8e2

	# Map the mimetype to the corresponding image. The writer may
	# choose the best image from these candidates. The special key * is
	# set if there is only single candidate to be used by a writer.
	# The special key ? is set for nonlocal URIs.

[mgeier]

Thanks for merging, and thanks for the additional information, that's very helpful!