wp_img_add_srcset_and_sizes function: Make the fallback set the width variable

[jaspermdegroot]

Passing the width to wp_get_attachment_image_sizes() prevents a call to image_get_intermediate_size() so we can improve performance by setting the $width variable if metadata is used to get the image size.

Related #34430. See https://core.trac.wordpress.org/attachment/ticket/34430/34430.2.diff which removes the $width argument from wp_get_attachment_image_sizes() and uses $size_array (array( width, height)) instead.

[azaozz]

Duplicate of #34430. Please see the patches there, they do this and a lot of other optimizations.