Limit the size of images included in srcset attributes

[joemcgill]

Since #33641, all available intermediate sizes of an image that are the same aspect ratio will be included in the image's srcset attribute.

In some cases, this may cause users to download unnecessarily large images. For example, if the original image is using the large size in the src attribute and the only larger option available for retina devices is an uncompressed full-size image, a browser could end up downloading a very large file.

While we can't filter images based on the file size, we can set an upper limit on the width of images that are included in srcset attributes by default and allow theme authors to filter the value as needed.

[joemcgill]

Add $max_srcset_width and default to 1600px

[azaozz]

In 35358:

Responsive Images: limit the size of images included in srcset attributes. Introduce max_srcset_image_width filter to adjust the limit.

Props joemcgill.
Fixes #34341.

[aduth]

@azaozz , I suspect there may be a typo in your r35358 - $contiue; should likely be continue;

https://core.trac.wordpress.org/browser/trunk/src/wp-includes/media.php?rev=35358#L959

[aduth]

Correctly continue to filter images wider than filter value

[azaozz]

In 35378:

Responsive Images: fix typo in r35358.

Props aduth.
See #34341.

[azaozz]

Replying to aduth:

Ugh, thought I fixed that from the "commit" dialog in Tortoisesvn, but it didn't get included in the current commit.