[renderer] Implement font attributes

- Again rename ImageCache to AssetCache and use it to cache fonts in addition to images
- Move AssetCache up the call stack to renderMetaTile, as it's used in more places in renderer
- Implement font-family, font-weight, font-style attributes

NB: Currently, mapcss grammar always skips whitespace in attribute values, so font-family: "DejaVu Sans" will be parsed as "DejaVuSans". However, cairo ignores it and works fine regardless. If it's needed, I have a patch that fixes grammar to not remove whitespace from attributes.