COGLayer: maintain previous zoom level while loading

[edeetee]

Problem

When zooming in, if the library goes to render the next zoom level, the existing zoom level disappears. This is jarring for a user and loses their context.

Screen.Recording.2026-02-23.at.9.06.00.AM.mov

Solution

When zooming in, maintain the existing zoom level in the background and let the higher resolution zoom appear on top.

[edeetee]

I believe this would be solved if #70 was implemented via the refinementStrategy

[kylebarron]

This is a duplicate of #190

Because we use a custom indexing scheme and not the web Mercator tiling scheme, the upstream deck.gl refinement strategies won't work out of the box. Feel free to discuss more on that issue if you'd like to work on a PR

[kylebarron]

Fixed in #354