Multi blend mode sys comp

We were just pulling blend mode lists from the device in st/oxr, instead of having them pass thru the compositor, despite the compositor having an idea of supported blend mode. That has been extended to an array of supported blend modes (in preference order), and the compositor now has a place to modify that list. @slitcch can you test this with your northstar as well as something opaque, to make sure I didn't break it? I don't have my NS set up.