avformat/flvdec: implement support for parsing ModEx data

libavformat/flv.h

@@ -130,6 +130,7 @@ enum {
     PacketTypeMetadata              = 4,
     PacketTypeMPEG2TSSequenceStart  = 5,
     PacketTypeMultitrack            = 6,
+    PacketTypeModEx                 = 7,
 };
 
 enum {
@@ -139,6 +140,10 @@ enum {
     AudioPacketTypeMultitrack         = 5,
 };
 
+enum {
+    PacketModExTypeTimestampOffsetNano = 0,
+};
+
 enum {
     AudioChannelOrderUnspecified = 0,
     AudioChannelOrderNative      = 1,