sub-optimal or even buggy non-scroll encoding

some encodings are both picture encodings and video encodings (ie: 'jpeg' and 'webp'),
so we should not be excluding them from the 'non-video' list.

the bug could be reproduced by connecting with '--encodings=scroll,webp --encoding=scroll'

xpra/server/window/window_video_source.py

@@ -232,7 +232,7 @@ def add(enc, encode_fn):
         #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:
         enc_options = set(self.server_core_encodings) & set(self._encoders.keys())
-        nv_common = (enc_options & set(self.core_encodings)) - set(self.video_encodings)
+        nv_common = enc_options & set(self.core_encodings) & set(self.picture_encodings)
         self.non_video_encodings = preforder(nv_common)
         log(f"init_encoders() server core encodings={self.server_core_encodings}, client core encodings={self.core_encodings}")
         log(f" video encodings={self.video_encodings}")