Mark allow-overlap symbols visible even outside of collision grid.

[ChrisLoer]

Fixes issue #7172, port of mapbox/mapbox-gl-native#12698.

The problem was that if allow-overlap: true symbols were placed while they were completely outside the collision grid (more than 100px past the edge of the viewport), their default opacity would get overwritten with a "not placed", and then they'd fade in once they came on-screen and got placed.

The solution is to force them to be visible even if they're not included in the collision grid. It seems like we should be able to just force them to be marked as "placed", but we don't want them to take up space in the collision grid.

This is unfortunately an extra cost in the inner placement loop, but it shouldn't be too much. FWIW, nothing show up in the Paint benchmark, but that would only catch a very large regression:

• 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
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

/cc @ansis

[ansis]

👍