only enable an encoding if we have an encoder for it

xpra/server/window/window_video_source.py

@@ -142,7 +142,9 @@ def init_encoders(self):
             self.non_video_encodings = ()
             self.common_video_encodings = ()
             return
-        self.video_encodings = tuple(x for x in self.video_helper.get_encodings() if x in self.server_core_encodings)
+        #make sure we actually have encoders for these:
+        enc_options = set(self.server_core_encodings) & set(self._encoders.keys())
+        self.video_encodings = tuple(x for x in self.video_helper.get_encodings() if x in enc_options)
         video_enabled = []
         for x in self.video_encodings:
             self.append_encoder(x, self.video_encode)
@@ -152,7 +154,7 @@ def init_encoders(self):
         self.add_encoder("auto", self.video_encode)
         #these are used for non-video areas, ensure "jpeg" is used if available
         #as we may be dealing with large areas still, and we want speed:
-        nv_common = (set(self.server_core_encodings) & set(self.core_encodings)) - set(self.video_encodings)
+        nv_common = (enc_options & set(self.core_encodings)) - set(self.video_encodings)
         self.non_video_encodings = tuple(x for x in PREFERRED_ENCODING_ORDER
                                          if x in nv_common)
         self.common_video_encodings = tuple(x for x in PREFERRED_ENCODING_ORDER