Renderer/writer support for osmc:symbol

[malasiot]

osmc:symbol in hiking route relations provide a textual definition of a graphic symbol. Rendering of these symbols may be performed in various ways.

• at rendering time by extending the style file to include src elements with value of the form symbol:/. In this case the symbol is rendered on the fly and cached. The symbol specifier may be used to render also other stock symbols like circle, square etc. We have to check if this breaks some renderers.
• at map creation time by generating suitable tags as those supported in elevate4 styles. This has the disadvantage that only nodes are supported. This can be done by adding a special command that will generate the required keys from an input key of the current feature
• at map creation as above but generate also a set of SVG images with filenames corresponding to the osmc tag e.g. osmc:symbol=green:white:green_bar -> green-white-green_bar.svg. Also generate the corresponding rule file.

[malasiot]

The first approach has been implemented. In particular the src attribute is now parsed and occurences of ${tag} are replaced by the value of the tag or empty string if it occurs in the feature. The resulting src for osmc symbols should then be "symbol:osmc:". During rendering the descriptor is parsed and renderer to a recording surface that is also cached. Renderers that encounter this symbol (tested in Mobac) fail silently (i.e. show nothing).

The second approach is also easy to implement. TODO.