Multiple symbols in the same tile can share the same crossTileID

[ChrisLoer]

This is the GL JS version of mapbox/mapbox-gl-native#10844 (comment). You can reproduce by modifying the "chinese.html" map to give every one of the points the same text label. Then by starting at z0 and zooming in, you can get to the state where all of the points are marked as duplicate against the first point.

The fix we plan is:

• Maintain a set of crossTileIDs used by each zoom level
• On detecting duplicates, check to see if the about-to-be-copied crossTileID is already in use for your zoom layer. If so, move on to the next potential duplicate. If no more potential duplicates are found, assign a new crossTileID
• On removing a bucket from the index, remove the set of crossTileIDs used by that bucket.

Example scenario imagining a z0 tile with symbols A and B, and a z1 tile with symbols A', B', and C' (all at the same location and with the same "key"):

• A' matches with A
• A' sees that the id from A is not used and copies it
• B' matches with A
• B' sees that the id from A is used and continues the search
• B' matches with B
• B' sees that the id from B is not used and copies it
• C' matches with A
• C' sees that the id from A is used and continues the search
• C' matches with B
• C' sees that the id from B is used and continues the search
• C' reaches the end of the search without copying and gets assigned a new id
  both A and B are duplicate and don't get rendered
  both A' and B' are instantly visible (since they used the opacities from A and B)
  C' fades in

/cc @ansis @friedbunny

[ChrisLoer]

Fixed with #5994.