decoder_api: pass "seekable" flag to decoder_initialized()

Don't pass the "seekable" flag with every decoder_data() invocation.
Since that flag won't change within the file, it is enough to pass it
to decoder_initialized() once per file.

src/decoder_thread.c

@@ -76,9 +76,6 @@ static void decodeStart(void)
 		}
 	}
 
-	/* for http streams, seekable is determined in input_stream_buffer */
-	dc.seekable = inStream.seekable;
-
 	if (dc.command == DECODE_COMMAND_STOP) {
 		input_stream_close(&inStream);
 		return;