wp_upload_dir is cached when using switch_to_blog

Ran into an issue on a large multisite when switching between blogs. `wp_upload_dir` is cached for the whole script execution, which means switching between blogs or filtering the `wp_upload_dir` filter will not have an effect on WP Thumb. 

We cache wpthumb for performance reasons as it is a rather slow function, however I think we might have to remove this as it's causing bugs.