Support hit tolerance in webgl layers

Is your feature request related to a problem? Please describe. I have troubles selecting thin linestring geometries on webgl layers, because the webgl vector renderer doesn't support hit tolerance.

Describe the solution you'd like

The only solution I can think of with regards to how the current webgl hit detection works is by scanning the circle around the requested coordinate. I think this should be fast enough for typical hit tolerances (10px or so) since it's just reading from the cached pixels data. But I wanted to propose / discuss this before creating a PR.