Fix ffmpeg H.264 pipe probing

Author: bonnieking

src/ac_training_lab/picam/device.py

@@ -180,6 +180,10 @@ def start_stream(ffmpeg_url, stream_key):
         "+genpts",
         "-f",
         "h264",
+        "-framerate",
+        "15",
+        "-video_size",
+        "854x480",
         "-i",
         "pipe:0",
         "-c:v",