Allow setClickableAreas to take weird objects

That is, objects that are not proper objects as WebKit's JSCValue can
understand them.

example/page.html

@@ -65,11 +65,7 @@ <h1>Welcome to the future!</h1>
         ;(async () => {
             // Make the magic square clickable.
             let square = document.getElementById('magic-square')
-            let squareBounds = square.getBoundingClientRect()
-            await Hudkit.setClickableAreas([
-                {x: squareBounds.x, y: squareBounds.y, width:
-                    squareBounds.width, height: squareBounds.height}
-            ])
+            await Hudkit.setClickableAreas([ square.getBoundingClientRect() ])
 
             // Clicks flip its text between uppercase and lowercase.
             let state = true

main.c

@@ -844,6 +844,9 @@ int main(int argc, char **argv) {
 "\n    return new Promise((resolve, reject) => {"
 "\n      const id = nextCallbackId++"
 "\n      window.Hudkit._pendingCallbacks[id] = { resolve, reject }"
+"\n      rectangles = rectangles.map(r => {"
+"\n         return { x: r.x, y: r.y, width: r.width, height: r.height }"
+"\n      })"
 "\n      window.webkit.messageHandlers.setClickableAreas.postMessage({id, rectangles})"
 "\n    })"
 "\n  },"