Undefined variable notices in inc/class-tachyon.php on line 363

I'm receiving a lot of undefined variables notices when using WP 5.3.2 with Tachyon, it caused by the `compact( ... 'size' )` variable in the `tachyon_post_image_args` hook.

```
[21-Dec-2019 10:47:54 UTC] PHP Notice: compact(): Undefined variable: size in /chassis/vendor/humanmade/tachyon-plugin/inc/class-tachyon.php on line 363
[21-Dec-2019 10:47:54 UTC] PHP Stack trace:
[21-Dec-2019 10:47:54 UTC] PHP 1. {main}() /chassis/index.php:0
[21-Dec-2019 10:47:54 UTC] PHP 2. require() /chassis/index.php:19
[21-Dec-2019 10:47:54 UTC] PHP 3. require_once() /chassis/wordpress/wp-blog-header.php:19
[21-Dec-2019 10:47:54 UTC] PHP 4. include() /chassis/wordpress/wp-includes/template-loader.php:98
[21-Dec-2019 10:47:54 UTC] PHP 5. get_template_part() /chassis/content/themes/koji/singular.php:11
[21-Dec-2019 10:47:54 UTC] PHP 6. locate_template() /chassis/wordpress/wp-includes/general-template.php:168
[21-Dec-2019 10:47:54 UTC] PHP 7. load_template() /chassis/wordpress/wp-includes/template.php:671
[21-Dec-2019 10:47:54 UTC] PHP 8. require() /chassis/wordpress/wp-includes/template.php:724
[21-Dec-2019 10:47:54 UTC] PHP 9. the_content() /chassis/content/themes/koji/content.php:23
[21-Dec-2019 10:47:54 UTC] PHP 10. apply_filters() /chassis/wordpress/wp-includes/post-template.php:253
[21-Dec-2019 10:47:54 UTC] PHP 11. WP_Hook->apply_filters() /chassis/wordpress/wp-includes/plugin.php:206
[21-Dec-2019 10:47:54 UTC] PHP 12. Tachyon::filter_the_content() /chassis/wordpress/wp-includes/class-wp-hook.php:288
[21-Dec-2019 10:47:54 UTC] PHP 13. compact() /chassis/vendor/humanmade/tachyon-plugin/inc/class-tachyon.php:363
```

The site in question is using smart media from Altis/media 2.x if that is likely to be having an effect.