Fix wp_editor_set_quality to support setting quality for both loaded and saved image types

[azaozz]

Introduce $output_mime_type in WP_Image_Editor and set it on saving the image if different from the original image's mime-type. Add tests.

Trac ticket: https://core.trac.wordpress.org/ticket/53667

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[azaozz]

Also introduces WP_Image_Editor::get_default_quality( $mime_type ) to be able to have different default quality for different image formats.

Currently the quality is (re)set and the wp_editor_set_quality filter runs only once when the output image type is different from the input image type. Can be changed to run every time on saving an image, for example when creating image sub-sizes.

[draganescu]

@azaozz is this done in https://core.trac.wordpress.org/changeset/51712 ?