Make decoder push frames to sinks

Now that screen implements the packet sink trait, make decoder push
packets to the sinks without depending on the concrete sink types.

app/src/scrcpy.c

@@ -318,7 +318,7 @@ scrcpy(const struct scrcpy_options *options) {
             file_handler_initialized = true;
         }
 
-        decoder_init(&decoder, &video_buffer);
+        decoder_init(&decoder);
         dec = &decoder;
     }
 
@@ -382,6 +382,8 @@ scrcpy(const struct scrcpy_options *options) {
         }
         screen_initialized = true;
 
+        decoder_add_sink(&decoder, &screen.frame_sink);
+
         if (options->turn_screen_off) {
             struct control_msg msg;
             msg.type = CONTROL_MSG_TYPE_SET_SCREEN_POWER_MODE;