change video code format to support chrome, but unsupport safari

demo/mask_detection/mask_detection.py

@@ -32,7 +32,7 @@ name = "./result/1-mask_detection.mp4"
 width = 1280
 height = 720
 fps = 30
-fourcc = cv2.VideoWriter_fourcc(*'mp4v')
+fourcc = cv2.VideoWriter_fourcc(*'vp90')
 writer = cv2.VideoWriter(name, fourcc, fps, (width, height))
 
 maskIndex = 0

demo/mask_detection/web/index.php

 <?php
-$json_string = file_get_contents(".\\video\\result\\2-mask_detection.json"); 
+$json_string = file_get_contents(".\\video\\result\\2-mask_detection.json");
 // 用参数true把JSON字符串强制转成PHP数组 
 $data_o = json_decode($json_string, true);
 ?>