feat(camera-prop): add auto option

Passing `auto` requests the rear camera by default and will
fallback to whatever kind of camera is installed otherwise. This
is the same behavior `rear` had before. `rear` now force requests
the rear camera.

The makes `rear` behavior more intuitive especially when dealing
switching between turning the camera on and off. This now means
switching between values `auto` and `off` instead of `rear` and
`off`.

BREAKING CHANGE: `rear` option for camera prop now force requests
rear camera. New `auto` option now implements old `rear` behavior.

Author: gruhn

src/components/QrcodeStream.vue

@@ -16,10 +16,7 @@
         class="pause-frame"
       ></canvas>
 
-      <canvas
-        ref="trackingLayer"
-        class="tracking-layer"
-      ></canvas>
+      <canvas ref="trackingLayer" class="tracking-layer"></canvas>
 
       <div class="overlay">
         <slot></slot>
@@ -40,10 +37,10 @@ export default {
   props: {
     camera: {
       type: String,
-      default: "rear",
+      default: "auto",
 
       validator(camera) {
-        return ["rear", "front", "off"].includes(camera);
+        return ["auto", "rear", "front", "off"].includes(camera);
       }
     },
 
@@ -102,9 +99,13 @@ export default {
       };
 
       switch (this.camera) {
-        case "rear":
+        case "auto":
           base.video.facingMode = { ideal: "environment" };
 
+          return base;
+        case "rear":
+          base.video.facingMode = { exact: "environment" };
+
           return base;
         case "front":
           base.video.facingMode = { exact: "user" };