next: add API for Canvas' plugin-props #74

Currently we have the following API for adding/overwriting props:

const T = createT(THREE, [xrEvents()])
return <T.Mesh onXRSelect={...} />

return <Entity from={...} plugins={[xrEvents()]} />

But this only sets props to the objects in the scene.
There is currently no API for setting Canvas plugin-props.

For the xr-events plugin (see #65) it would be handy if we could add additional props to the <Canvas/> component too.

Idea for API on the consuming side:

const [T, Canvas] = createT.withCanvas(THREE, [xrEvents()])
return (
  <Canvas onXRSelect={...} >
    <T.Mesh onXRSelect={...} />
  </Canvas>
)

Not sure yet how the authoring should look like, specifically the question if you want to add a prop only to Canvas, how do you filter for it as there is no corresponding three.js class?