Implement custom Refinement Strategy in `COGLayer`

[kylebarron]

When zooming in, we should render based on the lower-resolution data until the higher resolution data loads.

See https://deck.gl/docs/api-reference/geo-layers/tile-layer#refinementstrategy

Even though the default value of that is best-available, I figure that their implementation is probably tied to Web Mercator indexing, and since we don't have web mercator indexing, we probably have to implement a custom refinementStrategy

cc @batpad

[kylebarron]

It turns out this is a little complex because the upstream

• We should define a parents (with an s) attribute on TileHeader2D that we dynamically set on the upstream tile header object.
• Then we'll have to change the upstream refinement strategies for our multiple parents