Photon: srcset to use custom thumbnail sizes

[kraftbj]

If there are existing custom thumbnails at the matching size, we should use those instead.

Fixes #2919
Enhances #2885 See #2804

[kraftbj]

That said, I'm not sure if this approach is better/worse than something like 1ffcec2 or neither.

[kraftbj]

This is no good. You can have multiple custom thumbs of the same width. Assuming no Core changes, the other commit method is probably better. Let Core decide which image sizes to use and roll from there.

[kraftbj]

Core Slack chatter about if this approach seems sane to @joemcgill and asking about viability of a Core patch to make the dims easier to grab: https://wordpress.slack.com/archives/feature-respimg/p1447899460000167

[kraftbj]

Core ticket that would enhance the filter and would be beneficial to us: https://core.trac.wordpress.org/ticket/34736

[joemcgill]

If 34736 doesn't land before 4.4 I would recommend looking at an option similar to 5be08ef but matching the the $source['url'] against each $size['file'] from the meta array being passed to the filter. Alternately, you could use the $size_array to figure out the w/h ratio and use that to determine the correct height argument, since all the images in the srcset should have the same aspect ratio.

Not necessarily related, but you might be able to speed your filter up a bit by grabbing the full size image directly from $image_meta['file'], rather than runningstrip_image_dimensions_maybe()`.

[zinigor]

Why do we need these extra parameters here? I can't see any place where they are being used.

[kraftbj]

They can be removed from both lines. Added to make exploring options to fix faster, but in the end nothing helpful there for the final proposal. Thanks for catching it, can remove in a few hours if you don't get to it first.

[zinigor]

Read the discussion, tested the changes, this looks good except one thing I commented about earlier.