Header Image URL not Handled

[mikeselander]

When using Tachyon, a single header image doesn't have its image URL tachyoned.

This appears to be because a single header image is never passed through image_downsize - it is called directly from get_theme_mod where the URL is saved as text. Ref: https://github.com/WordPress/WordPress/blob/master/wp-includes/theme.php#L1052

It looks like this can be filtered via get_header_image_tag or, even better via theme_mod_header_image.

For randomized headers, we might want to filter wp_get_attachment_url as they pass through a different function. This doesn't appear to pass through image_downsize but I might be missing something.

Worth noting: the srcset for the header image is correctly techyoned, it's the main header image URL that is incorrect.