obs-outputs: Fix FLV muxing bug

The end of an FLV tag would contain the size of the tag, but the code
was erroneously including the end size value in addition, which it's not
supposed to do normally.

plugins/obs-outputs/flv-mux.c

@@ -164,7 +164,7 @@ bool flv_meta_data(obs_output_t *context, uint8_t **output, size_t *size,
 
 	s_write(&s, meta_data, meta_data_size);
 
-	s_wb32(&s, (uint32_t)serializer_get_pos(&s) - start_pos + 4 - 1);
+	s_wb32(&s, (uint32_t)serializer_get_pos(&s) - start_pos - 1);
 
 	*output = data.bytes.array;
 	*size   = data.bytes.num;
@@ -209,7 +209,7 @@ static void flv_video(struct serializer *s, int32_t dts_offset,
 	s_write(s, packet->data, packet->size);
 
 	/* write tag size (starting byte doesn't count) */
-	s_wb32(s, (uint32_t)serializer_get_pos(s) + 4 - 1);
+	s_wb32(s, (uint32_t)serializer_get_pos(s) - 1);
 }
 
 static void flv_audio(struct serializer *s, int32_t dts_offset,
@@ -242,7 +242,7 @@ static void flv_audio(struct serializer *s, int32_t dts_offset,
 	s_write(s, packet->data, packet->size);
 
 	/* write tag size (starting byte doesn't count) */
-	s_wb32(s, (uint32_t)serializer_get_pos(s) + 4 - 1);
+	s_wb32(s, (uint32_t)serializer_get_pos(s) - 1);
 }
 
 void flv_packet_mux(struct encoder_packet *packet, int32_t dts_offset,