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.
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git-svn-id: http://core.svn.wordpress.org/trunk@30778 1a063a9b-81f0-0310-95a4-ce76da25c4cd