Allow the pdf fallback_intermediate_image_sizes filter to process add_image_size() sizes.

[gitlost]

The fallback_intermediate_image_sizes filter added in #38594 only works for default get_option sizes and not user-defined add_image_size() sizes, unlike the intermediate_image_sizes_advanced filter. It'd be more useful if it could work with user-defined sizes also. My particular use case is adding a plugin-defined list thumbnail size, smaller than the default thumbnail size.

The following patch just copies the (before) logic used for intermediate_image_sizes_advanced to the (after) logic of fallback_intermediate_image_sizes. It includes a unittest.

[gitlost]

Process add_image_sizes() also.

[joemcgill]

Good catch @gitlost. You're correct that the filter isn't very useful for custom sizes if we don't take steps to check for them later. For the test, I would prefer a simple check that the filter was run, rather than making the filter conditional on the existence of the large size, in case that were to change later. What would you think of 39231.diff​?

[gitlost]

Looks good to me!

[joemcgill]

#39263 was marked as a duplicate.

[joemcgill]

In 39617:

Media: Allow PDF fallbacks filter to process custom sizes.

This fixes an oversight in [39246], which added a hook for filtering
the array of sizes used for PDF thumbnails, but failed to provide a way
for sizes added through add_image_size() to be processed.

Props gitlost.
Fixes #39231. See #38594.

[joemcgill]

Reopened for the 4.7 branch.

[pento]

In 39633:

Media: Allow PDF fallbacks filter to process custom sizes.

This fixes an oversight in [39246], which added a hook for filtering the array of sizes used for PDF thumbnails, but failed to provide a way for sizes added through add_image_size() to be processed.

Merge of [39617] to the 4.7 branch.

Props gitlost.
Fixes #39231. See #38594.