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/export.ts

@@ -42,6 +42,7 @@ export function exportToCanvas(
     elements,
     appState,
     null,
+    scale,
     rough.canvas(tempCanvas),
     tempCanvas,
     {