allow collisions with some symbols

obvious situation with strong improvement

http://www.openstreetmap.org/?mlat=49.4712&mlon=21.3811#map=15/49.4712/21.3811

![selection_011](https://cloud.githubusercontent.com/assets/899988/4475628/80032d84-496e-11e4-8cf1-80ca405b882a.png)

really high gate density with some improvements and some regressions, overall effect is IMHO positive (and new problem may be almost certainly fixed by adding halo to street names)

http://www.openstreetmap.org/#map=16/50.0484/19.9300

![selection_001](https://cloud.githubusercontent.com/assets/899988/4476284/a4236aca-4974-11e4-8e66-8527bedb697f.png)


Fixes parts of #323 and #964 

Likely also other symbols may benefit from similar change, but not all of them - see https://github.com/gravitystorm/openstreetmap-carto/issues/982#issuecomment-57062765