protocol/videoprotocol: add screenshot function

Extend the protocol with the screenshot function.

Signed-off-by: Rouven Czerwinski <[email protected]>

Author: Emantor

labgrid/protocol/videoprotocol.py

@@ -9,3 +9,7 @@ def get_qualities(self):
     @abc.abstractmethod
     def stream(self, quality_hint=None):
         raise NotImplementedError
+
+    @abc.abstractmethod
+    def screenshot(self, filename):
+        raise NotImplementedError