Advertise desktop decoder capabilities independently of encoders

tools/chatgui/src/call_video_decoder.h

@@ -10,6 +10,7 @@ public:
     ~CallVideoDecoder();
     bool decode(const call_video_frame& frame, QImage& image);
     void close();
+    static call_video_caps capabilities();
 private:
     struct Impl;
     std::unique_ptr<Impl> m;