WP_Image_Editor improvements.

* Make test() and supports_mime_type() static.
* Add required_methods argument to get_instance(). Allows requesting an implementation that has certain methods/capabilities.
* Whitespace cleanup

Props markoheijnen
see #6821


git-svn-id: https://develop.svn.wordpress.org/trunk@22510 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/includes/image-edit.php

@@ -237,7 +237,6 @@ function wp_stream_image( $image, $mime_type, $post_id ) {
 
 /**
  * Saves Image to File
- * @TODO: Add mime_type support to WP_Image_Editor
  *
  * @param string $filename
  * @param WP_Image_Editor $image