featuresAt() returns false positives with road labels

[peterqliu]

This test case shows black roads with black_label that are symbol_placement=line, and red roads with red_label that use default symbol placement.

When I hover over either road type to do a featuresAt({radius:10}), it returns road labels even when A) the cursor is clearly between labels, or B) the zoom level is too low for labels to even appear. Again, this happens regardless of symbol placement.

I would like to surface road labels only when (and where) the label is actually present.

cc/ @jfirebaugh @mourner