fix bug causing overzoomed tiles to disappear

[mollymerp]

Fixes #4564

For overzoomed tiles, TileCoord#cover returns TileCoords with incorrect z, so they dont appear to be within TileJSON.bounds for vector sources with that property, and thus (as of #4556 ) the SourceCache doesn't consider them part of visibleCoords (they get filtered out here) and so the source disappears if it's over its maxzoom

For instance in the case of the test case provided in #4564, the source has a maxzoom of 11, and the tiles within its bounds are [11/583/782, 11/584/782, 11/583/783, 11/584/784] but when you zoom to ZL 12, transform.coveringTiles returns [12/583/782, 12/584/782, 12/583/783, 12/584/784] which are not the correct tiles, and I guess doesn't cause problems besides this? Or maybe it's like this for a reason?

This PR doesn't fix that bc I'm scared to change TileCoord without understanding what's going on there. If the behavior of TileCoord is indeed a bug I can modify this PR to fix that. This PR is just a workaround to prevent over-zoomed vector sources with bounds from disappearing.

@anandthakker @jfirebaugh

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[mourner]

That's a known gotcha we've been meaning to fix — I attempted at some point but got stuck because it affects a lot of code throughout Mapbox GL. There should be an open ticket about this, trying to find it...

As for the fix, I think we should pass source.maxzoom to hasTile method and then just do Math.min(maxzoom, coord.zoom) inside — it's handled like this in other places currently. It's confusing, but will fix the issue while retaining the bounds filtering logic on overscaled zooms.

[mourner]

@mollymerp found the relevant tickets: #1912 and #2868

[mollymerp]

Thanks for the background @mourner - i've update the PR and will add a regression test next

[mourner]

Looks good! Can we use a smaller test though, e.g. 64x64? It should probably be enough to test the presence of an overzoomed tile.