Image block: fix lightbox srcset size

[sgomes]

What?

The image block currently generates the lightbox srcset without specifying a $size parameter to wp_get_attachment_image_srcset. This means that the $size parameter defaults to 'medium', per the method signature.

This PR changes that, to specify a larger size.

Why?

Some systems, like WordPress.com, rely on the $size parameter to determine the range of sizes to include in the srcset, to avoid including excessively large ones. In this case, since we're talking about a lightbox, the size that we're defaulting to ('medium') is not adequate, which leads to blurry images being shown, due to a lack of higher resolution options in the resulting srcset.

How?

This PR passes in an array with the image dimensions, when the image size is known. This ensures that the image can get displayed at the best possible resolution in the lightbox.

When the image size is unknown, it defaults to 'large' instead.

Testing Instructions

• Create an image block with a very large image attachment (above 2000px wide, if possible)
• Click the image to open the lightbox
• Analyse the srcset on the image in the lightbox
• Ensure that it includes large enough versions of the image

Testing Instructions for Keyboard

See the above instructions, but instead of clicking the image to open the lightbox, move focus to the image and hit the spacebar instead.

[luisherranz]

Reproduced the problem on a fresh installation and confirmed that this PR solves it.

Thanks, Sérgio!

[github-actions]

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Co-authored-by: sgomes <sergiomdgomes@git.wordpress.org>
Co-authored-by: luisherranz <luisherranz@git.wordpress.org>

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[sgomes]

Thank you for the review, @luisherranz! 👍

[github-actions]

I just cherry-picked this PR to the wp/7.0 branch to get it included in the next release: b9d2d1a