Only run placement for first layer per SymbolBucket

[ChrisLoer]

Native version of mapbox/mapbox-gl-js#6548.
Port of mapbox/mapbox-gl-js#6550.
This is a regression introduced by global symbol placement #10436.

The upshot of this bug is that you have two symbol layers that share the same layout properties, we try to run collision detection on both layers. They'll collide against each other and, depending on order, at least one and maybe both of them will be hidden.

/cc @ansis @ryanbaumann

[ChrisLoer]

Fixed in master (aka targeted for "chai" sdk releases) in #11775.

@ryanbaumann / @tmpsantos if this is a customer dependency in Qt (has there been a "boba" equivalent version of Qt yet?), let me know which branch it needs to go into and it should be a pretty easy cherry-pick.