Fix backward compatibility of tevkori_image_sizes_args

[jaspermdegroot]

To avoid having a lot of duplicate code to make both the tevkori_image_sizes_args filter and the $args parameter of tevkori_get_sizes() work, I used a global variable. Any objections?

[joemcgill]

The changes all look good to me. See my comment on 1d48433 about the use of backtics in PHP DocBlocks, which is a bit inconsistent both here and in Core itself. That said, I don't want to hold this up over a doc formatting issue.

[jaspermdegroot]

@joemcgill

As discussed it's not really clear where we to use backticks or single quotes. I will first check with DrewAPicture what the standard is and then create a patch for WP core and a PR for the plugin to make changes across all files (if needed).

[jaspermdegroot]

Test failures are because of solving a merge conflict so I am going to merge into dev.