Switch to a SVG for the parking icon.

Because this is such a simple icon, it's easy to write the SVG for it by hand.

This allows us to only use one symbol file for both parking and private parking, and set the colour in the stylesheet.
Fix parking icon alignment to pixel bounds

The round part of the P was half a pixel off.
Add XML prolog to SVG
Flatten out curve of P slightly, for better alignment with pixels.
Drop marker-height, as it duplicates the SVG and can lead to mistakes
Use a more generic name for the transportation blue
Use marker-clip for parking

This avoids duplicate Ps.
Split icon and text colours, since they're different
Use the variable for fuel too
Merge pull request #650 from pnorman/parking_svg

Replace parking icon with SVG