Move the logic from WP_Image_Editor::get_quality() to WP_Image_Editor::set_quality(), so that 'wp_editor_set_quality' and 'jpeg_quality' filters run when setting the default value.

props markoheijnen.
fixes #29856.

git-svn-id: https://develop.svn.wordpress.org/trunk@30788 602fd350-edb4-49c9-b593-d223f7449a82