Fix video headless pipeline

With mtkmdp element, the headless pipeline would stall if mtkmdp
connects directly to glbox. Add a workaround to fix the headless
pipeline until v4l2convert is enabled.

Change-Id: I788a8cf9bdf3a192561292e1b911e162241de69a
diff --git a/edgetpuvision/pipelines.py b/edgetpuvision/pipelines.py
index 1873c5d..950441e 100644
--- a/edgetpuvision/pipelines.py
+++ b/edgetpuvision/pipelines.py
@@ -110,10 +110,17 @@
        inference_pipeline(layout)],
     )
 
+# MTKMDP can't be used directly with glbox.
+# TODO: Remove the branching when v4l2convert is used.
 def video_headless_pipeline(filename, layout):
     return (
         [decoded_file_src(filename, layout.render_size),
-         Filter('glupload'),
+         Tee(name='t')],
+        [Pad('t'),
+         Queue(max_size_buffers=1, leaky='downstream'),
+         Filter('fakesink sync=False')],
+        [Pad('t'),
+         Queue(max_size_buffers=1, leaky='downstream'),
          inference_pipeline(layout)],
     )