Improve mobile clickability of boulders

[Humorloos]

see https://gis.stackexchange.com/questions/292963/openlayers-3-onclick-on-touch-device-not-triggering-perfectly

• https://openlayers.org/en/latest/apidoc/module-ol_Map-Map.html
• https://openlayers.org/en/latest/examples/hit-tolerance.html