fix(breakpoints): races in useViewportWidth (#114)

The main fix here is changing the initial state from null to
window.innerWidth, so that there isn't a useless transition from (for
example) `useDown('md') = true` to `useDown('md') = false` when the
application loads. Depending on the application, the rerendering penalty
can be heavy.

While we're here, notice that we can make a couple related changes:
First, since the initial state is set properly, we don't need a layout
effect, instead we can use a normal effect. Second, swap the order of
listener and setWidth to avoid the tiny race between them.

packages/core/src/breakpoints.js

@@ -31,16 +31,17 @@ function useThemeMaxValue(theme, key) {
 }
 
 export function useViewportWidth() {
-  const [width, setWidth] = React.useState(null)
-
-  React.useLayoutEffect(() => {
-    setWidth(window.innerWidth)
+  const [width, setWidth] = React.useState(typeof window === 'undefined' ? null : window.innerWidth)
 
+  React.useEffect(() => {
     function handleResize() {
       setWidth(window.innerWidth)
     }
 
+    // Add the listener, then setWidth to avoid race.
     window.addEventListener('resize', handleResize)
+    setWidth(window.innerWidth)
+
     return () => window.removeEventListener('resize', handleResize)
   }, [])