The fontInliner from the scratch-svg-renderer was changed recently to work entirely on strings, so we can use that instead of parsing/loading/possibly drawing/stringifying the entire SVG. 

This does not address the size-1 cache issue where the cache is invalidated any time there is more than 1 sprite. But that can be handled later.

Using the 4x CPU slowdown in chrome dev tools, on a simple costume that just has a single text element the getCostumeUrl time goes from 10ms to 1ms, very important for updating in a timely way.

Good catch @kchadha!

Cached url should be shared across watermark and sprite-selector-item for performance, address other review feedback.