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/components/App.tsx

@@ -2246,6 +2246,7 @@ export class App extends React.Component<any, AppState> {
       elements,
       this.state,
       this.state.selectionElement,
+      window.devicePixelRatio,
       this.rc!,
       this.canvas!,
       {