partial image caching optimization

[donpark]

While whole bot image can be cached, many use cases calls for partial image caching.

But each bot part has static (outline, shadow, etc.) and dynamic (coloring layer) elements which needs to be cached separately and handled differently during output.

To support optimizations like this generically, SVG elements needs to be tagged as hints to to drive caching. Again, this points to a need for plugin support in either parser or renderer.