Merge pull request #277 from tangrams/nvkelso/v1d8-changes

few other PR comments from Brett

fix collision_priority (was reversed); address PR comment; emphasize largest transit stations

remove janky layer

update comment per PR review

collision priority needs to check for null

add collision for water

pesky zoom 14 labels

rework new collision_priority function; add custom for roads and transit

remove superfilous visible: true

fix comment about POV

pull collision rank logic into separate global function

pull POV feature kind logic into separate global function

fix hgv tunnels

move toll and hgv to overlays