Use one SVG image and track loaded state w/ flag
Use one SVG image and track loaded state w/ flag

Instead of creating a new image every time `setSVG` is called and
checking whether that image is complete with `loaded`, reuse the same
image, override its `src`, & use `onload` instead of `addEventListener`
to replace the previous event listener. This is necessary to avoid race
conditions with the `_svgImageLoaded` flag.

LLK/scratch-render,LLK/scratch-render