fix(QrcodeStream): stop worker on destroy

The worker which is processing scanning jobs, is not terminated
when the component is destroyed. This was intended to be
guaranteed by the following mechanism:

1. Component is destroyed (for example with `v-if="false"`)
2. `beforeDestroy` hook is called
3. `this.destroyed = true`
4. computed property `shouldStream` becomes `false`
5. computed property `shouldScan` becomes `false`
6. `shouldScan` watcher calls `stopScanning`

Step 4 is never reached though. Probably becomes Vue won't
re-evaluate computed properties after the `beforeDestroy` hook
was called. So `stopScanning` is never called and the worker keeps
processing frames. Results are still handed to the component which
tries to paint to a canvas which doesn't exit anymore (since the
component is destroyed). Hence causing errors like

_Cannot read property 'getContext' of undefined_

See #85

Author: gruhn

src/components/QrcodeStream.vue

@@ -166,6 +166,7 @@ export default {
 
   beforeDestroy () {
     this.beforeResetCamera()
+    this.stopScanning()
     this.destroyed = true
   },
 
@@ -189,14 +190,13 @@ export default {
     },
 
     startScanning () {
-      // this.stopScanning()
-
       const detectHandler = result => {
         this.onDetect(
           Promise.resolve({ source: 'stream', ...result })
         )
       }
 
+      // this.stopScanning()
       this.stopScanning = keepScanning(this.cameraInstance, {
         detectHandler,
         locateHandler: this.onLocate,