Include 4th value, $is_intermediate, when preflighting `image_downsize`.

[peterwilsoncc]

The filter image_downsize expects an array of four values, Tachyon::filter_image_downsize() returns an array of three values, this causes an error in the AJAX response for query-attachment/s.

The forth value should indicate whether the image is an intermediate size, this is true if the response of image_get_intermediate_size() is truthy or the size is set as thumbnail. Tachyon will need some more advanced logic for custom sizes and named sizes defined after the image was originally uploaded.

I expect we could use $width < $full_width || $height < $full_height but I haven't dug into the code fully.

Following up to #22 (PR that closed #10).

[peterwilsoncc]

@joemcgill Are you able to take a look at the merged pull request for adding $is_intermediate to our downsize filter and suggest if we should set it to true whenever $width < $full_width || $height < $full_height?

Thanks boss.