WP_Image_Editor: the last stand.

 * Have wp_get_image_editor() rather than WP_Image_Editor::get_instance(). Having static factory methods would be less confusing if there weren't also static methods tied to individual editor implementations.
 * Lazy-load the WP_Image_Editor base class and editor implementations.
 * Have WP_Image_Editor_GD::supports_mime_type() actually check which types it supports.
 * Deprecate gd_edit_image_support() in favor of wp_image_editor_supports().

props DH-Shredder, scribu, markoheijnen. fixes #22356. see #6821.



git-svn-id: https://core.svn.wordpress.org/trunk@22817 1a063a9b-81f0-0310-95a4-ce76da25c4cd