Collision detection for symbol features is broken

[brunoabinader]

While debugging #6055, I noticed that symbol features in which placement calculation is performed - icon-ignore-placement is set to false - all features are rendered, including ones that should have been hidden due to collision with other visible features.

This is probably caused by a bug in CollisionTile implementation - which currently contains a few discrepancies from GL JS implementation - e.g. #6410 (comment).

/cc @jfirebaugh @ansis

[brunoabinader]

Other particularity I've noticed is that this issue mostly happens when we're on a fractional zoom level - I'm attaching a unit test to this to clarify.

[brunoabinader]

It turns out most of the issues I've perceived while debugging collision detection were caused either by symbol annotations not having a buffer on each neighbor tile to improve collision detection or duplicated collision results caused by a corner case when a symbol anchor is placed on tile edges.

This only leaves some issues related with perspective collision detection, which are handled in #6630.