symbols: replace simple non-SJJB PNG icons with SVG

Trying to replace all PNG symbols that do not come from the SJJB iconset. Related to #1108, which tries to replace all simple shaped symbols with SVG. There should not be any visual changes.

The following symbols are affected:
* gate
* lift gate
* mini roundabout
* peak (and volcano)
* power tower
* spring
* traffic light
don't use stroke, mapnik doesn't support it. at least not the version I was using.