Implement pattern fill for ways and areas (image, fill-image mapcss attributes)

- Rename IconCache to ImageCache, as it now is not only used for icons
- Use it to store fill patterns and implement fill-image mapcss attribute
I've had a thought that it may store Cairo::SurfacePatterns for fills (in addition to images for icons), but a microbenchmark showed that construction of pattern from image is very fast so it's not needed.
- Implement image mapcss attribute to fill ways as well

Obsoletes #18