Extract components for handling OSD + tile sources.

[cbeer]

This PR extracts components from OpenSeadragonViewer that manage the integration between OSD + react. By expressing the tile source as a react component, we're able to take advantage of react's internal rendering smarts instead of having to managing what may have changed.

[marlo-longley]

This looks great. Thank you Chris! Small comments --

1. needs a rebase

2. my main question which could be due to lack of understanding is -- if we can reduce the number of references to viewer and rely more on your new OpenSeadragonViewerContext. In my reading we have:

• setViewer in the parent component's state that gets passed down
• apiRef in the parent which is also holding a viewer and uses OSD methods like fitBounds
• the viewerRef in the child OpenSeadragonComponent
• also the viewer held in OpenSeadragonViewerContext

Something to think about (this also just helped me understand to write it out) -- and maybe for down the road not adding more work to this PR.

[cbeer]

I think your analysis is good. I don't understand the difference between apiRef + viewer in the component state and why we may or may not need both (I think we're using it as a hack to force a re-render when we initialize the viewer?), and I think the viewer prop could be replaced with a context consumer eventually (although we send it through to plugins which may need to be updated too).