Stop using getTransform (#950)

* Stop using getTransform

Fixes #861

The original motivation behind this is to make it work with Firefox. But it also helped make the code more intentional.

Test Plan:
- Create one square, select it, zoom in repeatedly, make sure that it zooms centered in the screen and everything looks good
- Scroll at various zoom levels, things look good
- Export a small scene at 1x and 3x, make sure the background is properly set and look good

* fix selection element

src/scene/index.ts

@@ -17,4 +17,4 @@ export {
   hasText,
 } from "./comparisons";
 export { createScene } from "./createScene";
-export { getZoomOrigin, getZoomTranslation, getNormalizedZoom } from "./zoom";
+export { getZoomOrigin, getNormalizedZoom } from "./zoom";