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

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).